Patent Publication Number: US-2020276867-A1

Title: Pneumatic Tire

Description:
FIELD OF THE TECHNOLOGY 
     The present technology relates to a pneumatic tire. 
     BACKGROUND OF THE TECHNOLOGY 
     Some known pneumatic tires have specified dimensions at predetermined positions to ensure desired performance. For example, a pneumatic tire described in Japan Patent No. 5567839 has a specified ratio between a tread width and the distance between an end portion of a belt layer and an outermost edge of a carcass to suppress diameter growth of a tread portion. Additionally, a run-flat radial tire described in Japan Unexamined Patent Publication No. 2015-205583 has a specified ratio between a tire cross-sectional height and an overlapping width in the tire axial direction between a maximum belt width layer and a side reinforcing rubber layer to prevent rim disengagement. 
     Here, in recent years, there has been an increasing need to increase a designated internal pressure to reduce tire rolling resistance. On the other hand, as the internal pressure of the pneumatic tire increases, the rigidity of a ground contact surface increases, making the ground contact surface less likely to deform in a case where a foreign material is treaded. This is likely to degrade shock burst resistance performance corresponding to resistance to a shock burst resulting from treading of the foreign material. Thus, reducing the rolling resistance without degrading the shock burst resistance performance has been very difficult. 
     SUMMARY 
     The present technology provides a pneumatic tire that can provide both shock burst resistance performance and reduced rolling resistance in a compatible manner. 
     A pneumatic tire is provided including at least one carcass layer, a belt layer disposed outward, in a tire radial direction, of a portion of the carcass layer located in the tread portion, and a tread rubber layer disposed outward, in the tire radial direction, of the belt layer in the tread portion, the tread portion being provided with a main groove extending in a tire circumferential direction, and a plurality of land portions defined by the main groove, and in the tread portion, assuming that a center region is a region where a center land portion included in the land portions and corresponding to the land portion closest to a tire equatorial plane is located, shoulder regions are each a region between a position corresponding to 85% of a width of the belt layer in a tire lateral direction and an end portion of the belt layer in the tire lateral direction, and intermediate regions are each a region between the center region and a corresponding one of the shoulder regions, a relationship between a tire average thickness Gc in the center region and a tire average thickness Gsh in the shoulder regions and a tire average thickness Gm in the intermediate regions being within a range of 1.05≤(Gc/Gsh)≤1.35 and satisfying a relationship Gc≥Gm&gt;Gsh. 
     Additionally, preferably, in the pneumatic tire described above, in the tread portion, a relationship between an average thickness Tc of the tread rubber layer located outward, in the tire radial direction, from the belt layer in the center region, and an average thickness Tsh of the tread rubber layer located outward, in the tire radial direction, from the belt layer in the shoulder regions, and an average thickness Tm of the tread rubber layer located outward, in the tire radial direction, from the belt layer in the intermediate regions is within a range of 1.2≤(Tc/Tsh)≤1.9 and satisfies a relationship Tc≥Tm&gt;Ts. 
     Additionally, preferably, in the pneumatic tire described above, in the tread portion, a relationship between an average actual rubber thickness Vc of the tread rubber layer in the center region and an average actual rubber thickness Vsh of the tread rubber layer in the shoulder regions is within a range of 1.6≤(Vc/Vsh)≤2.5. 
     Additionally, preferably, in the pneumatic tire described above, at least one of the land portions located in the center region and the intermediate regions is formed as a projection-shaped land portion for which a relationship between a thickness Te at an end portion position of the land portion in the tire lateral direction and a thickness Tp at a central position of the land portion in the tire lateral direction is Tp&gt;Te. 
     Additionally, preferably, in the pneumatic tire described above, the projection-shaped land portion is formed in an arc shape in which a ground contact surface representing an outer contour line in a tire meridian cross-sectional view bulges outward in the tire radial direction, and a relationship between a radius of curvature RR of the arc and a radius of curvature TR of an arc forming a tread profile is within a range of 0.1≤(RR/TR)≤0.4. 
     Additionally, preferably, in the pneumatic tire described above, the belt layer bulges inward in the tire radial direction at a position located inward, in the tire radial direction, of the at least one of the land portions located in the center region and the intermediate regions. 
     Additionally, preferably, in the pneumatic tire described above, in the tread portion, a relationship between a minimum thickness Tg of a rubber thickness between the belt layer and a groove bottom of the main groove defining the center land portion and the average thickness Tc of the tread rubber layer located outward, in the tire radial direction, from the belt layer in the center region is within a range of 0.12≤(Tg/Tc)≤0.4. 
     Additionally, preferably, in the pneumatic tire described above, a portion of rubber forming the tread rubber layer that is contained in the center region is 10 MPa or more and 16 MPa or less in modulus of 300% elongation. 
     Additionally, preferably, in the pneumatic tire described above, a circumferential narrow groove extending in the tire circumferential direction is formed in at least one of the shoulder regions located on opposite sides in the tire lateral direction. 
     Additionally, preferably, in the pneumatic tire described above, a portion of the carcass layer located in the shoulder region bulges toward a tire inner surface in an internal-pressure uninflated state. 
     Additionally, preferably, in the pneumatic tire described above, a belt reinforcing layer is disposed outward of the belt layer in the tire radial direction, and a larger number of pieces of the belt reinforcing layer are layered at a position of the center region than at positions other than the position of the center region. 
     A pneumatic tire according to an embodiment of the present technology produces the effect of enabling provision of both shock burst resistance performance and reduced rolling resistance in a compatible manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a meridian cross-sectional view illustrating a main portion of a pneumatic tire according to a first embodiment. 
         FIG. 2  is a detailed view of a portion A of  FIG. 1 . 
         FIG. 3  is an explanatory diagram of the thickness of a tread rubber layer. 
         FIG. 4  is a perspective view of a main portion of a tread portion and is an explanatory diagram illustrating the actual rubber thickness of a tread rubber layer. 
         FIG. 5  is an explanatory diagram illustrating that a projection on a road surface is treaded by the pneumatic tire according to the first embodiment. 
         FIG. 6  is a detailed cross-sectional view illustrating a main portion of a pneumatic tire according to a second embodiment. 
         FIG. 7  is a detailed cross-sectional view illustrating a main portion of a pneumatic tire according to a third embodiment. 
         FIG. 8  is a detailed cross-sectional view illustrating a main portion of a pneumatic tire according to a fourth embodiment. 
         FIG. 9  is a detailed cross-sectional view illustrating a main portion of a pneumatic tire according to a fifth embodiment. 
         FIG. 10  is a detailed cross-sectional view illustrating a main portion of a pneumatic tire according to a sixth embodiment. 
         FIG. 11A  is a table showing the results of performance evaluation tests of pneumatic tires. 
         FIG. 11B  is a table showing the results of performance evaluation tests of pneumatic tires. 
         FIG. 11C  is a table showing the results of performance evaluation tests of pneumatic tires. 
     
    
    
     DETAILED DESCRIPTION 
     Pneumatic tires according to embodiments of the present technology are described in detail below with reference to the drawings. However, the present technology is not limited by the embodiment. Constituents of the following embodiments include elements that are substantially identical or that can be substituted and easily conceived by one skilled in the art. 
     First Embodiment 
     Hereinafter, “tire radial direction” refers to the direction orthogonal to the rotation axis (not illustrated) of a pneumatic tire  1 . “Inward in the tire radial direction” refers to the direction toward the rotation axis in the tire radial direction. “Outward in the tire radial direction” refers to the direction away from the rotation axis in the tire radial direction. “Tire circumferential direction” refers to the circumferential direction with the rotation axis as the center axis. Additionally, “tire lateral direction” refers to a direction parallel with the rotation axis. “Inward in the tire lateral direction” refers to a direction toward a tire equatorial plane (tire equator line) CL in the tire lateral direction. 
     “Outward in the tire lateral direction” refers to a direction away from the tire equatorial plane CL in the tire lateral direction. “Tire equatorial plane CL” refers to a plane that is orthogonal to the rotation axis of the pneumatic tire  1  and that passes through the center of the tire width of the pneumatic tire  1 . The position of the tire equatorial plane CL in the tire lateral direction coincides with a tire lateral direction center line corresponding to a central position of the pneumatic tire  1  in the tire lateral direction. “Tire width” is the width in the tire lateral direction between components located outermost in the tire lateral direction, or in other words, the distance between the components that are the most distant from the tire equatorial plane CL in the tire lateral direction. “Tire equator line” refers to the line in the tire circumferential direction of the pneumatic tire  1  that lies on the tire equatorial plane CL. 
       FIG. 1  is a meridian cross-sectional view illustrating a main portion of a pneumatic tire  1  according to a first embodiment. In the pneumatic tire  1  illustrated in  FIG. 1 , in a tire meridian cross-sectional view, a tread portion  2  is disposed in an outermost portion in the tire radial direction, and includes a tread rubber layer  4  formed of a rubber composition. Additionally, a surface of the tread portion  2 , that is, a portion of the pneumatic tire that comes into contact with a road surface during traveling of a vehicle (not illustrated in the drawings) mounted with the pneumatic tire  1 , is formed as a ground contact surface  3 , which forms a part of contour of the pneumatic tire  1 . The ground contact surface  3  of the tread portion  2  is provided with a plurality of main grooves  30  extending in the tire circumferential direction. The main grooves  30  define a plurality of land portions  20  in the surface of the tread portion  2 . In the first embodiment, four main grooves  30  are formed side by side in the tire lateral direction, and two of the four main grooves  30  are disposed on each of opposite sides of the tire equatorial plane CL in the tire lateral direction. In other words, the tread portion  2  is provided with a total of four main grooves  30  including two center main grooves  31  disposed on the opposite sides of the tire equatorial plane CL and two shoulder main grooves  32  disposed outward, in the tire lateral direction, of each of the two center main grooves  31 . 
     Note that “main groove  30 ” refers to a longitudinal groove that extends at least partially in the tire circumferential direction. In general, the main groove  30  has a groove width of  3  mm or more and a groove depth of  6  mm or more, and internally includes a tread wear indicator (skid sign) that indicates terminal stages of wear. In the first embodiment, the main groove  30  has a groove width of 9 mm or more and 12 mm or less and a groove depth of 7 mm or more and 8 mm or less, and is substantially parallel with the tire equator line (center line) where the tire equatorial plane CL and the ground contact surface  3  intersect. The main grooves  30  may extend linearly in the tire circumferential direction, or may be provided in a wave-like shape or a zigzag shape. 
     A center land portion  21  corresponds to the land portion  20  included in the land portions  20  defined by the main grooves  30  and located between the two center main grooves  31  and on the tire equatorial plane CL. Additionally, second land portions  22  each correspond to the land portion  20  located between the center main groove  31  and the shoulder main groove  32  that are adjacent to each other and disposed outward of the center land portion  21  in the tire lateral direction. Additionally, shoulder land portions  23  each correspond to the land portion  20  located outward of the second land portion  22  in the tire lateral direction and adjacent to the second land portion  22  via the shoulder main groove  32 . 
     Additionally, shoulder portions  5  are located at both outer ends of the tread portion  2  in the tire lateral direction, and sidewall portions  8  are each disposed inward of the shoulder portion  5  in the tire radial direction. In other words, the sidewall portions  8  are disposed at two positions on the opposite sides of the pneumatic tire  1  in the tire lateral direction, and form outermost exposed portions of the pneumatic tire  1  in the tire lateral direction. 
     Bead portions  10  are each located inward, in the tire radial direction, of the corresponding one of the sidewall portions  8  located on the opposite sides in the tire lateral direction. Similar to the sidewall portions  8 , the bead portions  10  are disposed at two positions on the opposite sides of the tire equatorial plane CL, that is, the bead portions  10  in a pair are disposed on opposite sides of the tire equatorial plane CL in the tire lateral direction. Each of the bead portions  10  is provided with a bead core  11 , and a bead filler  12  is provided outward of the bead core  11  in the tire radial direction. The bead core  11  is an annular member formed by winding, in a ring shape, a bead wire including a steel wire, and the bead filler  12  is a rubber member disposed outward of the bead core  11  in the tire radial direction. 
     Additionally, a belt layer  14  is provided inward of the tread portion  2  in the tire radial direction. The belt layer  14  has a multilayer structure in which at least two cross belts  141  and  142  are layered. The cross belts  141  and  142  are formed by coating, with coating rubber, a plurality of belt cords made of steel or an organic fiber material such as polyester, rayon, or nylon, and performing a rolling process on the coating rubber-covered belt cords, and the cross belts  141  and  142  have a belt angle within a predetermined range (for example, of 20° or more and 55° or less), the belt angle being defined as the inclination angle of each belt cord with respect to the tire circumferential direction. Additionally, the two cross belts  141  and  142  are different from each other in belt angle. Thus, the belt layer  14  is what is called a crossply structure in which the two cross belts  141  and  142  are layered with the inclination directions of the belt cords intersecting each other. The tread rubber layer  4  of the tread portion  2  is disposed outward, in the tire radial direction, of the belt layer  14  in the tread portion  2 . 
     A carcass layer  13  containing cords of radial plies is continuously provided inward of the belt layer  14  in the tire radial direction and in a portion of the sidewall portion  8  closer to the tire equatorial plane CL. The carcass layer  13  has a single layer structure made of one carcass ply or a multilayer structure made of a plurality of carcass plies, and extends between the pair of bead portions  10  on the opposite sides in the tire lateral direction in a toroidal form, forming the backbone of the tire. Specifically, the carcass layer  13  is disposed from one bead portion  10  to the other bead portion  10  located on either side in the tire lateral direction, and turns back outward in the tire lateral direction along the bead cores  11  at the bead portions  10 , wrapping around the bead cores  11  and the bead fillers  12 . The bead filler  12  is a rubber material disposed in a space formed outward of the bead core  11  in the tire radial direction by folding back the carcass layer  13  at the bead portion  10 . Additionally, the belt layer  14  is disposed in the carcass layer  13  extending between the pair of bead portions  10  and outward, in the tire radial direction, of a portion of the carcass layer  13  located in the tread portion  2 . Additionally, the carcass plies in the carcass layer  13  are made by coating, with coating rubber, a plurality of carcass cords made of steel or an organic fiber material such as nylon, polyester, or rayon and performing a rolling process on the coating rubber-covered carcass cords. A plurality of carcass cords forming carcass plies are disposed side by side forming a certain angle with respect to the tire circumferential direction along the tire meridian direction. 
     A rim cushion rubber  17  is disposed radially inward and laterally outward of the bead core  11  and turned back portion of the carcass layer  13  at the bead portion  10 . The rim cushion rubber  17  is the contact surface of the bead portion  10  against the rim flange. Additionally, an innerliner  16  is formed inward of the carcass layer  13  or inward of the carcass layer  13  in the pneumatic tire  1 , along the carcass layer  13 . The innerliner  16  forms a tire inner surface  18  corresponding to an inner surface of the pneumatic tire  1 . 
       FIG. 2  is a detailed view of a portion A of  FIG. 1 . Assuming that the tread portion  2  includes a center region Ac located in the center of the tread portion  2  in the tire lateral direction, shoulder regions Ash located at opposite ends of the tread portion  2  in the tire lateral direction, and intermediate regions Am each located between the center region Ac and the corresponding shoulder region Ash, a relative relationship between the tire average thicknesses of the respective regions satisfies a predetermined relationship. Among these regions, the center region Ac is a region where the center land portion  21  included in the land portions  20  and located closest to the tire equatorial plane CL is positioned. Specifically, in a meridian cross-sectional view of the pneumatic tire  1 , assuming that center region boundary lines Lc are each a line extended perpendicularly to the tire inner surface  18  from an intersection point  24  between a portion of the groove wall  35  of the center main groove  31  defining the center land portion  21  which portion is located closer to the center land portion  21  and the ground contact surface  3  representing an outer contour line of the center land portion  21  located outward in the tire radial direction, the center region Ac corresponds to a region located between the two center region boundary lines Lc located on the opposite sides of the center land portion  21  in the tire lateral direction. 
     Note that when the center main groove  31  undulates in the tire lateral direction by bending or curving in the tire lateral direction while extending in the tire circumferential direction, the center region Ac is defined in the widest range in the tire lateral direction. In other words, in a case where the center main groove  31  undulates in the tire lateral direction, the center region boundary lines Lc defining the center region Ac each correspond to a line extended, perpendicularly to the tire inner surface  18 , from the intersection point  24  between the ground contact surface  3  and a portion of the groove wall  35  of the center main groove  31  defining the center land portion  21  which portion is located outermost in the tire lateral direction on the tire circumferential direction. 
     Additionally, the shoulder region Ash is a region between a position P corresponding to 85% of the width of the belt layer  14  in the tire lateral direction and an end portion  144  of the belt layer  14  in the tire lateral direction. Specifically, in a meridian cross-section view of the pneumatic tire  1 , assuming that shoulder region boundary lines Lsh correspond to lines extended, perpendicularly to the tire inner surface  18 , from a position P corresponding to 85% of the width of a widest belt  143  included in the plurality of cross belts  141  and  142  of the belt layer  14  and from an end portion  144  of the widest belt  143 , the shoulder region Ash corresponds to a region located between the two shoulder region boundary lines Lsh. The shoulder regions Ash defined as described above are defined by the opposite sides the tire equatorial plane CL in the tire lateral direction, and are located on the opposite sides of the tire equatorial plane CL in the tire lateral direction. 
     In the first embodiment, the cross belt  141  included in the two cross belts  141  and  142  of the belt layer  14  and located inward in the tire radial direction is larger than the other cross belt  142  in width in the tire lateral direction, and the cross belt  141  located inward in the tire radial direction corresponds to the widest belt  143 . 
     Additionally, the position P corresponding to  85 % of the width of the widest belt  143  in the tire lateral direction is the position of an end portion of a region accounting for 85% of the width of the widest belt  143  in the tire lateral direction in a case where the 85% region is evenly distributed on the opposite sides in the tire lateral direction with the center of the 85% region corresponding to the center of the widest belt  143  in the tire radial direction or the position of the tire equatorial plane CL. Thus, the spacing between the position P corresponding to 85% of the width of the widest belt  143  in the tire lateral direction and the end portion  144  of the widest belt  143  is the same on the opposite sides of the tire equatorial plane CL in the tire lateral direction. 
     Additionally, the intermediate region Am is a region between the center region Ac and the shoulder region Ash. In other words, the intermediate region Am is located on both sides of the center region Ac in the tire lateral direction, the boundary of the intermediate region Am located inward in the tire lateral direction is defined by the center region boundary line Lc, and the boundary of the intermediate region Am located outward in the tire lateral direction is defined by the shoulder region boundary line Lsh. 
     The center region Ac, the shoulder regions Ash, and the intermediate regions Am are specified to have shapes obtained with the pneumatic tire  1  mounted on a regular rim and inflated to a regular internal pressure. Here, “regular rim” refers to a “standard rim” defined by JATMA (Japan Automobile Tyre Manufacturers Association, Inc.), a “Design Rim” defined by TRA (The Tire and Rim Association, Inc.), or a “Measuring Rim” defined by ETRTO (European Tire and Rim Technical Organization). “Regular internal pressure” refers to a “maximum air pressure” defined by JATMA, the maximum value in “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” defined by TRA, or “INFLATION PRESSURES” defined by ETRTO. 
     In the tread portion  2 , a relative relationship between the tire average thicknesses in the center region Ac and the shoulder regions Ash and the intermediate regions Am satisfies a predetermined relationship. In this case, the tire average thickness is the average value, for each region, of the tire thickness, which is a thickness from the ground contact surface  3  to the tire inner surface  18 , the ground contact surface  3  representing an outer contour line corresponding to an outer contour line of the land portion  20  in the tire radial direction in a tire meridian cross-sectional view. In other words, the tire average thickness Gc in the center region Ac is the average value of the distance from the ground contact surface  3  to the tire inner surface  18  in the center region Ac, and the tire average thickness Gsh in the shoulder regions Ash is the average value of the distance from the ground contact surface  3  to the tire inner surface  18  in the shoulder regions Ash, and the tire average thickness Gm in the intermediate regions Am is the average value of the distance from the ground contact surface  3  to the tire inner surface  18  in the intermediate regions Am. 
     Although the center main groove  31  and the shoulder main groove  32  are located in the intermediate regions Am of the tread portion  2 , the tire average thickness Gm in the intermediate regions Am is calculated on the assumption that these main grooves  30  are not present. In other words, for the tire thickness at the position of the main groove  30 , the tire average thickness Gm in the intermediate regions Am is calculated with a distance from an imaginary line  25  to the tire inner surface  18  considered as the tire thickness at the position of the main groove  30 , the imaginary line  25  being obtained by extending the ground contact surface  3  of the land portions  20  on the opposite sides of the main groove  30  in the tire lateral direction. 
     The tire average thickness of each of the regions may be calculated, for each of the center region Ac, the shoulder regions Ash, and the intermediate regions Am of the tread portion  2  in the meridian cross-section of the pneumatic tire  1 , by dividing the cross-sectional area of the region by the width of the region. For example, the tire average thickness Gc of the center region Ac is calculated by dividing the cross-sectional area of the center region Ac by the distance between two center region boundary lines Lc defining the center region Ac. In a case where the two center region boundary lines Lc are inclined with respect to each other, the tire average thickness Gc of the center region Ac is calculated by dividing the cross-sectional area of the center region Ac by the distance at an intermediate position between the position of the ground contact surface  3  and the position of the tire inner surface  18  on each center region boundary line Lc. The tire average thickness Gsh in the shoulder regions Ash and the tire average thickness Gm in the intermediate regions Am are similarly calculated by dividing the cross-sectional area of each of the regions by the distance between the shoulder region boundary lines Lsh defining the region or the distance between the center region boundary line Lc and the shoulder region boundary line Lsh. 
     In the tread portion  2 , a relationship between the tire average thickness Gc in the center region Ac and the tire average thickness Gsh in the shoulder regions Ash calculated as described above is within a range of 1.05≤(Gc/Gsh)≤1.35. Furthermore, in the tread portion  2 , the tire average thickness Gc in the center region Ac, the tire average thickness Gsh in the shoulder regions Ash, and the tire average thickness Gm in the intermediate regions Am satisfies a relationship Gc≥Gm&gt;Gsh. Note that the relationship between the tire average thickness Gc in the center region Ac and the tire average thickness Gsh in the shoulder regions Ash is preferably within a range of 1.08≤(Gc/Gsh)≤1.20. 
       FIG. 3  is an explanatory diagram of the thickness of the tread rubber layer  4 . In the tread portion  2 , not only the tire average thickness for each region, but also the relative relationship between the thicknesses of the tread rubber layer  4  at the respective regions satisfies the predetermined relationship. In other words, in the tread portion  2 , a relationship between the average thickness Tc of the tread rubber layer  4  located outward, in the tire radial direction, from the belt layer  14  in the center region Ac and the average thickness Tsh of the tread rubber layer  4  located outward, in the tire radial direction, from the belt layer  14  in the shoulder regions Ash is within a range of 1.2≤(Tc/Tsh)≤1.9. Furthermore, in the tread portion  2 , for the average thickness Tc of the tread rubber layer  4  located outward, in the tire radial direction, from the belt layer  14  in the center region Ac, the average thickness Tsh of the tread rubber layer  4  located outward, in the tire radial direction, from the belt layer  14  in the shoulder regions Ash, and the average thickness Tm of the tread rubber layer  4  located outward, in the tire radial direction, from the belt layer  14  in the intermediate regions Am, a relationship Tc≥Tm&gt;Ts is satisfied. Note that, in  FIG. 3 , the tread rubber layer  4  in each of the regions is hatched. Additionally, a relationship between the average thickness Tc of the tread rubber layer  4  in the center region Ac and the average thickness Tsh of the tread rubber layer  4  in the shoulder regions Ash is preferably within a range of 1.4≤(Tc/Tsh)≤1.7. 
     The average thickness of the tread rubber layer  4  in each of the regions in this case is the average thickness in each region obtained on the assumption that the thickness of the tread rubber layer  4  is defined as the distance between the ground contact surface  3  and the cross belt  142  included in the cross belts  141  and  142  of the belt layer  14  and located outward in the tire radial direction. Additionally, the average thickness Tm of the tread rubber layer  4  in the intermediate regions Am is calculated, similarly to the tire average thickness Gm in the intermediate regions Am, on the assumption that the main grooves  30  are not present. In other words, for the thickness of the tread rubber layer  4  at the position of the main groove  30 , the average thickness Tm of the tread rubber layer  4  in the intermediate regions Am is calculated with a distance from the imaginary line  25  to the cross belt  142  considered as the thickness of the tread rubber layer  4  at the position of the main groove  30 , the imaginary line  25  being obtained by extending the ground contact surface  3  of the land portions  20  on the opposite sides of the main groove  30  in the tire lateral direction, the cross belt  142  being included in the two cross belts  141  and  142  and located outward in the tire radial direction. 
     Like the tire average thickness, the thickness of the tread rubber layer  4  defined as described above may be calculated, for each of the center region Ac, the shoulder regions Ash, and the intermediate regions Am of the tread portion  2  in the meridian cross-section of the pneumatic tire  1 , by dividing the cross-sectional area of the tread rubber layer  4  in the region by the width of the region. 
     In addition, at least a portion of the rubber forming the tread rubber layer  4  that is contained in the center region Ac is 10 MPa or more and 16 MPa or less in modulus of 300% elongation. Note that the modulus of 300% elongation is measured in a tensile test at 23° C. in conformance with JIS (Japanese Industrial Standard) K6251 (using dumbbell No. 3) and indicates tensile stress at 300% elongation. 
     Furthermore, in the tread portion  2 , for an actual rubber thickness corresponding to the thickness of the tread rubber layer  4  obtained taking into account the grooves formed in the tread portion  2 , the relative relationship between the regions satisfies a predetermined relationship. In other words, for an average actual rubber thickness corresponding to the actual rubber thickness calculated for each region, the relative relationship between the regions satisfies a predetermined relationship.  FIG. 4  is a perspective view of a main portion of the tread portion  2 , and is an explanatory diagram of the actual rubber thickness of the tread rubber layer  4 . The main grooves  30  are formed in the tread portion  2 , and besides the main grooves  30 , extending in the tire circumferential direction, grooves such as lug grooves  40  that extend in the tire lateral direction are formed. The average thickness of the tread rubber layer  4  described above is the thickness of the tread rubber layer  4  calculated with no consideration of these grooves and on the assumption that no grooves are present, whereas the average actual rubber thickness of the tread rubber layer  4  is the thickness of the tread rubber layer  4  calculated on the assumption that no rubber forming the tread rubber layer  4  is present in the groove portions. Thus, the average actual rubber thickness of the tread rubber layer  4  in each of the regions is a thickness calculated by dividing the actual volume of the tread rubber layer  4  in each region excluding the grooves such as the main grooves  30  and the lug grooves  40  by the area of a portion of the tire inner surface  18  located in each of the regions. 
     For example, the average actual rubber thickness Vc of the tread rubber layer  4  in the center region Ac is calculated by dividing the volume of the tread rubber layer  4  in the center region Ac excluding the grooves by the area of a portion of the tire inner surface  18  located in the center region Ac. The area of the portion of the tire inner surface  18  located in the center region Ac corresponds to the area of a portion of the tire inner surface  18  that extends in the tire circumferential direction between the two center region boundary lines Lc defining the center region Ac. 
     Additionally, the average actual rubber thickness Vsh of the tread rubber layer  4  in the shoulder region Ash is calculated by dividing the volume of the tread rubber layer  4  in the shoulder region Ash excluding the grooves by the area of a portion of the tire inner surface  18  located in the shoulder region Ash. The area of the portion of the tire inner surface  18  located in the shoulder region Ash corresponds to the area of a portion of the tire inner surface  18  that extends in the tire circumferential direction between two shoulder region boundary lines Lsh defining the shoulder region Ash. 
     In the tread portion  2 , a relationship between the average actual rubber thickness Vc of the tread rubber layer  4  in the center region Ac and the average actual rubber thickness Vsh of the tread rubber layer  4  in the shoulder regions Ash calculated as described above is within a range of 1.6≤(Vc/Vsh)≤2.5. 
     Note that the average actual rubber thickness of the tread rubber layer  4  in each of the regions may be calculated by cutting out the tread rubber layer  4  from the pneumatic tire  1  for each of the regions, calculating the volume based on the mass of a cutout tread rubber layer  4  and the specific gravity of the rubber forming the tread rubber layer  4 , and dividing the calculated volume by the area of a portion of the tire inner surface  18  located in the region. 
     When mounted on a vehicle, the pneumatic tire  1  according to the first embodiment is mounted on a rim wheel R (see  FIG. 5 ) by fitting the rim wheel R to the bead portion  10 , and is inflated by filling the inside with air before the mounting on the vehicle. The pneumatic tire  1  according to the first embodiment is used in a state in which the internal pressure during inflation is relatively high, and specifically, is used at an internal pressure of 250 kPa or more and 290 kPa or less. When a vehicle equipped with the pneumatic tire  1  travels, the pneumatic tire  1  rotates with a portion of the ground contact surface  3  located below coming into contact with the road surface. The vehicle travels by transferring a driving force and a braking force to the road surface or generating a turning force due to the frictional force between the ground contact surface  3  and the road surface. 
     For example, in a case of traveling on a dry road surface, a vehicle equipped with the pneumatic tire  1  travels by transferring a driving force and a braking force to the road surface or generating a turning force mainly due to the frictional force between the ground contact surface  3  and the road surface. Additionally, during traveling on a wet road surface, water between the ground contact surface  3  and the road surface enters the grooves such as the main grooves  30  and the lug groove  40  and is drained by these grooves. Accordingly, the ground contact surface  3  comes into easy contact with the road surface, and the friction force between the ground contact surface  3  and the road surface allows the vehicle to travel as desired. 
     Additionally, while the vehicle is traveling, the pneumatic tire  1  is subjected to a load associated with the weight of the vehicle body, acceleration and deceleration, and turning, leading to a heavy load acting in the tire radial direction. The load is mainly borne by the air with which the pneumatic tire  1  is filled, but is received not only by the air inside the pneumatic tire  1  but also by the tread portion  2  and the sidewall portion  8 . Specifically, the sidewall portion  8  transfers a load between the bead portion  10  to which the rim wheel R is fitted and the tread portion  2 , and the tread portion  2  transfers a load between the sidewall portion  8  and the road surface. Thus, a heavy load acts on the sidewall portion  8  and the tread portion  2  while the vehicle is traveling, and the sidewall portion  8  and the tread portion  2  receive the load while bending mainly in the tire radial direction. 
     Additionally, while the vehicle is traveling, the pneumatic tire  1  rotates, and thus the position of the ground contact surface  3  that comes into contact with the road surface continuously moves in the tire circumferential direction. Consequently, positions on the sidewall portion  8  and the tread portion  2  that are deflected by the load while the vehicle is traveling also move in the tire circumferential direction. Thus, while the vehicle is traveling, the pneumatic tire  1  rotates with the positions on the sidewall portion  8  and the tread portion  2  in the tire circumferential direction sequentially repeatedly deflected. 
     Here, the deflection of the sidewall portion  8  and the tread portion  2  in this manner prevents rotation of the pneumatic tire  1  in contact with the road surface, and acts as strong resistance during rotation of the pneumatic tire  1 . Thus, significant deflection of the pneumatic tire  1  during rotation of the pneumatic tire  1  increases what is called rolling resistance, corresponding to the resistance during rotation of the pneumatic tire  1 . 
     In contrast, the pneumatic tire  1  according to the first embodiment is used under an internal pressure of 250 kPa or more and 290 kPa or less, that is, the pneumatic tire  1  is used under a relatively high internal pressure. Accordingly, much of the load acting on the pneumatic tire  1  can be received by the internal pressure. Thus, the sidewall portion  8  and the tread portion  2  are less likely to be deflected by the load acting on the pneumatic tire  1 , and the resistance during rotation of the pneumatic tire  1  can be reduced that is caused by the deflection of the sidewall portion  8  and the tread portion  2 . This enables a reduction in rolling resistance during rotation of the pneumatic tire  1 . 
     Additionally, on the road surface on which the vehicle travels, projections such as stones that project from the road surface may be present, and while the vehicle is traveling, the tread portion  2  of the pneumatic tire  1  may tread such a projection. At this time, in a case where the sidewall portion  8  and the tread portion  2  are insignificantly deflected due to the high internal pressure, the pneumatic tire  1  fails to absorb a change in the shape of the road surface caused by the presence of the projection, and the projection may penetrate the tread portion  2  of the pneumatic tire  1 . In other words, in a case where the pneumatic tire  1  inflated to a high internal pressure treads the projection on the road surface, the projection may penetrate the tread portion  2  due to the insignificant deflection of the sidewall portion  8  and the tread portion  2 , leading to a shock burst. 
     In contrast, the pneumatic tire  1  according to the first embodiment has an increased tire average thickness Gc in the center region Ac and a reduced tire average thickness Gsh in the shoulder regions Ash, allowing a possible shock burst to be suppressed in a case of an increased internal pressure.  FIG. 5  is an explanatory diagram illustrating that a projection  105  on a road surface  100  is treaded by the pneumatic tire  1  according to the first embodiment. The pneumatic tire  1  according to the first embodiment has an increased tire average thickness Gc in the center region Ac to enable an increase in strength at break at or near the center of the tread portion  2  in the tire lateral direction. Thus, even in a case where the pneumatic tire  1  treads the projection  105  on the road surface  100  at or near the center region Ac, penetration of the tread portion  2  by the projection  105  can be suppressed. Additionally, a reduced tire average thickness Gsh in the shoulder regions Ash allows the shoulder regions Ash to deform preferentially in a case where the projection  105  is treaded by the vicinity of the center region Ac of the tread portion  2 , allowing the shoulder regions Ash to deform easily in the direction in which the vicinity of the center region Ac is separated from the road surface  100 . Accordingly, the pressure from the projection  105  acting on the tread portion  2  can be reduced, allowing suppression of penetration the projection  105  by the tread portion  2 . Thus, a possible shock burst can be suppressed that is caused by treading of the projection  105  during traveling of the vehicle. 
     Specifically, in the tread portion  2  of the pneumatic tire  1  according to the first embodiment, the relationship between the tire average thickness Gc in the center region Ac and the tire average thickness Gsh in the shoulder regions Ash is within the range of 1.05≤(Gc/Gsh)≤1.35. Thus, with the rolling resistance reduced, a possible shock burst can be suppressed. In other words, in a case where the relationship between the tire average thickness Gc in the center region Ac and the tire average thickness Gsh in the shoulder regions Ash is (Gc/Gsh)&lt;1.05, the tire average thickness Gc in the center region Ac is excessively small, hindering the strength at break of the center region Ac from being increased. Alternatively, the tire average thickness Gsh in the shoulder regions Ash is excessively large, making the shoulder regions Ash less likely to deform. In a case where the projection  105  is treaded by the tread portion  2 , the shoulder regions Ash are less likely to deform in the direction in which the vicinity of the center region Ac is separated from the road surface  100 . 
     Additionally, in a case where the relationship between the tire average thickness Gc in the center region Ac and the tire average thickness Gsh in the shoulder regions Ash is (Gc/Gsh)&gt;1.35, the tire average thickness Gc in the center region Ac is excessively large and the tire average thickness Gsh in the shoulder regions Ash is excessively small, leading to a significant difference in contact length between the central region and both end regions of the contact patch shape of the ground contact surface  3  in the tire lateral direction. This makes the rolling resistance more likely to increase. In other words, a contact patch shape having a large ground contact length near the center in the tire lateral direction and a small contact length near both ends in the tire lateral direction means that the deflection of the tread portion  2  differs between the central region and both end regions in the tire lateral direction. The deflection is more significant in the central region than in both end regions. Accordingly, the deflection of the tread portion  2  occurring in a case where the ground contact surface  3  comes into contact with the ground concentrates in the central region of the tread portion  2  in the tire lateral direction, and only this portion is significantly deflected. Thus, the significant deflection of the central region of the tread portion  2  in the tire lateral direction makes the rolling resistance likely to increase. 
     In contrast, in a case where the relationship between the tire average thickness Gc in the center region Ac and the tire average thickness Gsh in the shoulder regions Ash is within the range of 1.05≤(Gc/Gsh)≤1.35, then with the significant deflection of only the central region of the tread portion  2  in the tire lateral direction suppressed, the strength at break of the center region Ac can be ensured and easiness of deformation of the shoulder regions Ash can be ensured, in a case where the ground contact surface  3  comes into contact with the ground. Accordingly, with the rolling resistance reduced, a possible shock burst can be suppressed, allowing shock burst resistance performance to be improved. 
     Furthermore, in the tread portion  2 , the tire average thickness Gc in the center region Ac, the tire average thickness Gsh in the shoulder regions Ash, and the tire average thickness Gm in the intermediate regions Am satisfy the relationship Gc≥Gm&gt;Gsh. Thus, the tire thickness of the tread portion  2  can be continuously varied from the center region Ac to the intermediate region Am and the shoulder region Ash. Accordingly, the out-of-plane bending rigidity of the tread portion  2  can be continuously varied along the tire lateral direction. Thus, stress concentration that is caused by deflection of the tread portion  2  associated with treading of the projection  105  by the tread portion  2  can be suppressed. Additionally, the tire average thickness Gsh is smallest in the shoulder regions Ash, which are subjected to a heavy energy loss during rotation of the pneumatic tire  1  due to relatively significant deflection in a case where the ground contact surface  3  comes into contact with the ground. This enables a reduction in resistance offered in a case where the shoulder regions Ash are deflected. Thus, the energy loss during rotation of the pneumatic tire  1  can be reduced, enabling a reduction in rolling resistance. As a result, both shock burst resistance performance and reduced rolling resistance can be provided in a compatible manner. 
     Additionally, in the tread portion  2 , the relationship between the average thickness Tc of the tread rubber layer  4  in the center region Ac and the average thickness Tsh of the tread rubber layer  4  in the shoulder regions Ash is within the range of 1.2≤(Tc/Tsh)≤1.9. Thus, with the rolling resistance reduced, a possible shock burst can be suppressed. In other words, if the relationship between the average thickness Tc of the tread rubber layer  4  in the center region Ac and the average thickness Tsh of the tread rubber layer  4  in the shoulder region Ash is (Tc/Tsh)&lt;1.2, the average thickness Tc of the tread rubber layer  4  in the center region Ac is too thin, so there is a risk that the strength at break of the center region Ac will be difficult to increase. In another case, because the average thickness Tsh of the tread rubber layer  4  in the shoulder region Ash is too thick, there is a risk that the shoulder regions Ash are less likely to deform when the projections  105  are treaded. Additionally, in a case where the relationship between the average thickness Tc of the tread rubber layer  4  in the center region Ac and the average thickness Tsh of the tread rubber layer  4  in the shoulder regions Ash is (Tc/Tsh)&gt;1.9, the average thickness Tc of the tread rubber layer  4  in the center region Ac is excessively large, and the average thickness Tsh of the tread rubber layer  4  in the shoulder regions Ash is excessively small. The central region of the contact patch shape of the ground contact surface  3  in the tire lateral direction has a substantially larger contact length than both end regions of the contact patch shape of the ground contact surface  3  in the tire lateral direction. In this case, in a case where the ground contact surface  3  comes into contact with the ground, only the central region of the tread portion  2  in the tire lateral direction is likely to deflect significantly, and this may be likely to increase the rolling resistance. 
     In contrast, in a case where the relationship between the average thickness Tc of the tread rubber layer  4  in the center region Ac and the average thickness Tsh of the tread rubber layer  4  in the shoulder regions Ash is within the range of 1.2≤(Tc/Tsh)≤1.9, then with the significant deflection of only the central region of the tread portion  2  in the tire lateral direction suppressed, the strength at break of the center region Ac can be ensured and easiness of deformation of the shoulder regions Ash can be ensured, in a case where the ground contact surface  3  comes into contact with the ground. Accordingly, with the rolling resistance reduced, a possible shock burst can be suppressed, allowing shock burst resistance performance to be improved. 
     Furthermore, in the tread portion  2 , the average thickness Tc of the tread rubber layer  4  in the center region Ac, and the average thickness Tsh of the tread rubber layer  4  in the shoulder regions Ash, and the average thickness Tm of the tread rubber layer  4  in the intermediate region Am satisfy the relationship Tc≥Tm&gt;Ts. Thus, the thickness of the tread rubber layer  4  can be continuously varied from the center region Ac to the intermediate region Am and the shoulder region Ash. Accordingly, the out-of-plane bending rigidity of the tread portion  2  can be more reliably continuously varied along the tire lateral direction, and stress concentration can be more reliably suppressed that is caused by deflection of the tread portion  2  associated with treading of the projection  105  by the tread portion  2 . Additionally, the average thickness Tsh of the tread rubber layer  4  in the shoulder regions Ash is smallest, and thus the resistance offered in a case where the shoulder regions Ash are deflected can be more reliably reduced. Accordingly, energy loss during rotation of the pneumatic tire  1  can be reduced, enabling a reduction in rolling resistance. As a result, both shock burst resistance performance and reduced rolling resistance can be more reliably provided in a compatible manner. 
     Additionally, in the tread portion  2 , the relationship between the average actual rubber thickness Vc of the tread rubber layer  4  in the center region Ac and the average actual rubber thickness Vsh of the tread rubber layer  4  in the shoulder regions Ash is within the range of 1.6≤(Vc/Vsh)≤2.5. Thus, with the rolling resistance reduced, a possible shock burst can be suppressed. In other words, in a case where the relationship between the average actual rubber thickness Vc of the tread rubber layer  4  in the center region Ac and the average actual rubber thickness Vsh of the tread rubber layer  4  in the shoulder regions Ash is (Vc/Vsh)&lt;1.6, the average actual rubber thickness Vc of the tread rubber layer  4  in the center region Ac is excessively small. This may lead to difficulty in increasing the strength at break of the center region Ac. In another case, an excessively large average actual rubber thickness Vsh of the tread rubber layer  4  in the shoulder regions Ash may make the shoulder regions Ash less likely to deform in a case where the projection  105  is treaded. Additionally, in a case where the relationship between the average actual rubber thickness Vc of the tread rubber layer  4  in the center region Ac and the average actual rubber thickness Vsh of the tread rubber layer  4  of the shoulder region Ash is (Vc/Vsh)&gt;2.5, the average actual rubber thickness Vc of the tread rubber layer  4  in the center region Ac is too thick, and the average actual rubber thickness Vsh of the tread rubber layer  4  in the shoulder region Ash is too thin. Accordingly, there is a risk that the ground contact length at or near the center of the ground contact surface  3  in the tire lateral direction may be significantly longer than the ground contact length at or near both ends in the tire lateral direction. In this case, at the time of contact with the ground contact surface  3 , only the central region of the tread portion  2  in the tire lateral direction is likely to deflect significantly, and this may be likely to increase the rolling resistance. 
     In contrast, in a case where the relationship between the average actual rubber thickness Vc of the tread rubber layer  4  in the center region Ac and the average actual rubber thickness Vsh of the tread rubber layer  4  in the shoulder regions Ash is within the range of 1.6≤(Vc/Vsh)≤2.5, then with the significant deflection of only the central region of the tread portion  2  in the tire lateral direction suppressed, the strength at break of the center region Ac can be ensured and easiness of deformation of the shoulder regions Ash can be ensured, in a case where the ground contact surface  3  comes into contact with the ground. As a result, both shock burst resistance performance and reduced rolling resistance can be more reliably provided in a compatible manner. 
     In addition, a portion of the rubber forming the tread rubber layer  4  that is contained at least in the center region Ac is 10 MPa or more and 16 MPa or less in modulus of 300% elongation. Thus, with the strength of the tread rubber layer  4  ensured, the tread portion  2  can be appropriately deflected. In other words, in a case where the rubber contained in the center region Ac in the tread rubber layer  4  is less than 10 MPa in modulus of 300% elongation, a portion of the rubber located in the center region Ac of the tread rubber layer  4  may be excessively soft. In a case where the projection  105  is treaded by the central region of the tread portion  2  in the tire lateral direction, the projection  105  may penetrate the tread rubber layer  4 . In this case, the projection  105  penetrating the tread rubber layer  4  may reach the belt layer  14  and damage the belt layer  14 . Additionally, in a case where the rubber contained in the center region Ac in the tread rubber layer  4  is more than 16 MPa in modulus of 300% elongation, the tread portion  2  may be excessively difficult to deflect in a case where the projection  105  is treaded by the tread portion  2 . The protrusion of the projection  105  from the road surface  100  may be prevented from being absorbed by deflection of the tread portion  2 . In this case, even with the high strength of the tread rubber layer  4 , the projection  105  may penetrate the tread rubber layer  4  and damage the belt layer  14 . 
     In contrast, in a case where the rubber contained in the center region Ac in the tread rubber layer  4  is 10 MPa or more and 16 MPa or less in modulus of 300% elongation, then with the tread rubber layer  4  provided with a strength sufficient to allow suppression of penetration of the projection  105 , the tread portion  2  can be appropriately deflected to allow the protrusion of the projection  105  from the road surface  100  to be absorbed to some degree in a case where the projection  105  is treaded by the tread portion  2 . As a result, the shock burst resistance performance can be more reliably improved. 
     Second Embodiment 
     The pneumatic tire  1  according to a second embodiment has a configuration substantially similar to the configuration of the pneumatic tire  1  according to the first embodiment. However, the pneumatic tire  1  according to the second embodiment has the feature that at least one land portion  20  is formed as a protrusion-shaped land portion  26 . The other components are similar to the corresponding components of the first embodiment, and thus description of the components is omitted and the same reference numerals are used. 
       FIG. 6  is a detailed cross-sectional view of a main portion of the pneumatic tire  1  according to the second embodiment. In the pneumatic tire  1  according to the second embodiment, in the tread portion  2 , the relationship among the tire average thickness Gc in the center region Ac, the tire average thickness Gsh in the shoulder regions Ash, and the tire average thickness Gm in the intermediate regions Am is within the range of 1.05≤(Gc/Gsh)≤1.35 and satisfies the relationship Gc≥Gm&gt;Gsh, as is the case with the pneumatic tire  1  according to the first embodiment. 
     Additionally, in the pneumatic tire  1  according to the second embodiment, at least one of the land portions  20  located in the center region Ac and in the intermediate regions Am is formed as a protrusion-shaped land portion  26  in which a relationship between a thickness Te of the land portion  20  at the position of an end portion in the tire lateral direction and a thickness Tp of the land portion  20  at a central position in the tire lateral direction is Tp&gt;Te. In other words, the protrusion-shaped land portion  26  is thicker at the central position in the tire lateral direction than at the positions of both end portions in the tire lateral direction. The thickness in this case is the distance between the ground contact surface  3  and the tire inner surface  18  in a tire meridian cross-sectional view. 
     In the second embodiment, the center land portion  21  corresponds to the protrusion-shaped land portion  26 . The thickness Te of the center land portion  21  at the position of the end portion in the tire lateral direction corresponds to the distance between the tire inner surface  18  and an intersection point  24  between a groove wall  35  of the center main groove  31  defining the center land portion  21  and the ground contact surface  3  of the center main groove  31 . The thickness Tp at the central position in the tire lateral direction is a distance between the tire inner surface  18  and a central position  27 , in the tire lateral direction, of the ground contact surface  3  of the center land portion  21 . In the center land portion  21  corresponding to the protrusion-shaped land portion  26 , the relationship between the thickness Te at the position of the end portion in the tire lateral direction and the thickness Tp at the central position in the tire lateral direction is Tp&gt;Te, the thicknesses Te and Tp being defined as described above. 
     Additionally, the center land portion  21  is formed in an arc shape in which the ground contact surface  3  representing an outer profile line of the center land portion  21  in a tire meridian cross-sectional view bulges outward in the tire radial direction. Accordingly, the center land portion  21  is formed as the protrusion-shaped land portion  26  that is thicker at the central position in the tire lateral direction than the positions of both end portions in the tire lateral direction. 
     Additionally, in the center land portion  21 , the ground contact surface  3  is formed projecting outward, in the tire radial direction, from a tread profile PR corresponding to a contour line used as a reference for the ground contact surface  3  of the center land portion  21 . Note that the tread profile PR is a standard contour line for an internal-pressure uninflated state, and a comparison between the ground contact surface  3  of the center land portion  21  and the tread profile PR refers to a comparison between the shape of the ground contact surface  3  of the center land portion  21  in the internal-pressure uninflated state and the tread profile PR. 
     In this case, the tread profile PR refers to an arc that passes through at least three of the four opening ends E of the two main grooves  30  adjacent to the land portion  20  and opposite to each other in the tire lateral direction in a tire meridian cross-sectional view of the internal-pressure uninflated state, the center of the arc being located inward of the ground contact surface  3  in the tire radial direction, the arc being drawn with the maximum radius of curvature. In other words, the tread profile PR of the center land portion  21  is an arc that passes through at least three of the four open ends E of the two center main grooves  31  adjacent to opposite sides of the center land portion  21  in the tire lateral direction, and the center of the arc is an arc that is positioned inward in the tire radial direction of the ground contact surface  3  and is drawn with a maximum radius of curvature. 
     Additionally, for the ground contact surface  3  of the center land portion  21 , a radius of curvature RR of an arc corresponding to the shape of the ground contact surface  3  in a tire meridian cross-section view is smaller than a radius of curvature TR of the arc forming the tread profile PR. Specifically, the radius of curvature RR of the ground contact surface  3  of the center land portion  21  in a tire meridian cross-sectional view is within a range of 0.1≤(RR/TR)≤0.4 with respect to the radius of curvature TR of the tread profile PR. 
     Furthermore, in the tread portion  2 , a relationship between a minimum thickness Tg of the rubber thickness between the belt layer  14  and a groove bottom  36  of the center main groove  31  defining the center land portion  21  and the average thickness Tc of the tread rubber layer  4  located outward, in the tire radial direction, from the belt layer  14  in the center region Ac is within a range of 0.12≤(Tg/Tc)≤0.4. Note that the relationship between the minimum thickness Tg of the rubber thickness between the groove bottom  36  of the center main groove  31  and the belt layer  14  and the average thickness Tc of the tread rubber layer  4  in the center region Ac is preferably within a range of 0.15≤(Tg/Tc)≤0.25. 
     In the pneumatic tire  1  according to the second embodiment, the center land portion  21  is formed as the protrusion-shaped land portion  26  that is thicker at the central position in the tire lateral direction than at the positions of both end portions in the tire lateral direction. Thus, the strength against an external obstacle can be more reliably increased. Thus, even in a case where the projection  105  on the road surface  100  is treaded by the center land portion  21 , penetration of the center land portion  21  by the projection  105  can be more reliably suppressed. As a result, the shock burst resistance performance can be more reliably improved. 
     Additionally, the center land portion  21  corresponding to the protrusion-shaped land portion  26  is formed in an arc shape in which the ground contact surface  3  bulges outward in the tire radial direction, and the relationship between the radius of curvature RR of the ground contact surface  3  and the radius of curvature TR of the tread profile PR is within the range of 0.1≤(RR/TR)≤0.4. Thus, with the rolling resistance reduced, a possible shock burst can be suppressed. In other words, in a case where the radius of curvature RR of the ground contact surface  3  of the center land portion  21  with respect to the radius of curvature TR of the tread profile PR is represented as (RR/TR)&lt;0.1, the radius of curvature RR of the ground contact surface  3  is excessively small. Thus, the ground contact surface  3  of the center land portion  21  may excessively significantly bulge outward in the tire radial direction with respect to the tread profile PR. In this case, the central region, in the tire lateral direction, of the contact patch shape of the entire ground contact surface  3  of the tread portion  2  has a significantly larger contact length than both end regions of the contact patch shape in the tire lateral direction, making only the central region in the tire lateral direction likely to deflect significantly in a case where the tread portion  2  comes into contact with the ground. This may be likely to increase the rolling resistance. Additionally, in a case where the radius of curvature RR of the ground contact surface  3  of the center land portion  21  with respect to the radius of curvature TR of the tread profile PR is represented as (RR/TR)&gt;0.4, the radius of curvature RR of the ground contact surface  3  is excessively large. Thus, the ground contact surface  3  of the center land portion  21  may excessively insignificantly bulge outward in the tire radial direction. In this case, properly ensuring the tire thickness of the center region Ac is difficult, and increasing the strength at break of the center region Ac may be difficult. This may lead to difficulty in appropriately improving the shock burst resistance performance. 
     In contrast, in a case where the relationship between the radius of curvature RR of the ground contact surface  3  of the center land portion  21  and the radius of curvature TR of the tread profile PR is within the range of 0.1≤(RR/TR)≤0.4, then with the significant deflection of only the central region of tread portion  2  in the tire lateral direction suppressed, the strength at break of the center region Ac can be ensured in a case where the tread portion  2  comes into contact with the ground. As a result, both shock burst resistance performance and reduced rolling resistance can be more reliably provided in a compatible manner. 
     Additionally, in the tread portion  2 , the relationship between the minimum thickness Tg of the rubber thickness between the groove bottom  36  of the center main groove  31  and the belt layer  14  and the average thickness Tc of the tread rubber layer  4  of the center region Ac is within the range of 0.12≤(Tg/Tc)≤0.4. Thus, with energy loss during rotation of the pneumatic tire  1  reduced, significant local deformation of the tread portion  2  can be suppressed. In other words, in a case where the minimum thickness Tg of the rubber thickness between the groove bottom  36  of the center main groove  31  and the belt layer  14  with respect to the average thickness Tc of the tread rubber layer  4  of the center region Ac is represented as (Tg/Tc)&lt;0.12, the minimum thickness Tg between the center main groove  31  and the belt layer  14  is excessively small. Thus, in a case where the tread portion  2  treads the projection  105  and is subjected to bending displacement, excessively significant deformation may occur at the position of the center main groove  31 . In this case, the tread portion  2  deforms locally, and thus the tread portion  2  may be likely to be damaged, and improving the shock burst resistance performance may be difficult. Additionally, in a case where the minimum thickness Tg of the rubber thickness between the groove bottom  36  of the center main groove  31  and the belt layer  14  with respect to the average thickness Tc of the tread rubber layer  4  of the center region Ac is represented as (Tg/Tc)&gt;0.4, the minimum thickness Tg of the rubber thickness between the center main groove  31  and the belt layer  14  is excessively large. Thus, the energy loss during rotation of the pneumatic tire  1  is likely to increase, and reducing the rolling resistance may be difficult. 
     In contrast, in a case where the relationship between the minimum thickness Tg of the rubber thickness between the groove bottom  36  of the center main groove  31  and the belt layer  14  and the average thickness Tc of the tread rubber layer  4  of the center region Ac is within a range of 0.12≤(Tg/Tc)≤0.4, then with the energy loss during rotation of the pneumatic tire  1  reduced, significant local deformation of the tread portion  2  at the position of the center main groove  31  can be suppressed in a case where the projection  105  is treaded by the tread portion  2 . As a result, both shock burst resistance performance and reduced rolling resistance can be more reliably provided in a compatible manner. 
     Third Embodiment 
     The pneumatic tire  1  according to a third embodiment has a configuration substantially similar to the configuration of the pneumatic tire  1  according to the second embodiment. However, the pneumatic tire  1  according to the third embodiment has the feature that a portion of the belt layer  14  bulges inward in the tire radial direction. The other components are similar to the corresponding components of the second embodiment, and thus description of the components is omitted and the same reference numerals are used. 
       FIG. 7  is a detailed cross-sectional view of a main portion of the pneumatic tire  1  according to the third embodiment. In the pneumatic tire  1  according to the third embodiment, in the tread portion  2 , the relationship among the tire average thickness Gc in the center region Ac, the tire average thickness Gsh in the shoulder regions Ash, and the tire average thickness Gm in the intermediate regions Am is within the range of 1.05≤(Gc/Gsh)≤1.35 and satisfies the relationship Gc≥Gm&gt;Gsh, as is the case with the pneumatic tire  1  according to the second embodiment. Additionally, the center land portion  21  is formed as the protrusion-shaped land portion  26  that extends outward in the tire radial direction. 
     Furthermore, in the pneumatic tire  1  according to the third embodiment, the belt layer  14  bulges inward in the tire radial direction in the internal-pressure uninflated state at a position located inward, in the tire radial direction, of at least one of the land portions  20  located in the center region Ac and in the intermediate regions Am. In the third embodiment, the belt layer  14  bulges inward in the tire radial direction in the internal-pressure uninflated state at the position located inward of the center land portion  21  in the tire radial direction. In other words, the belt layer  14  is formed projecting toward the tire inner surface  18  in the internal-pressure uninflated state at the position located inward of the center land portion  21  in the tire radial direction. 
     Note that, with the belt layer  14  bulging inward in the tire radial direction, the carcass layer  13  and the innerliner  16  disposed inward of the belt layer  14  in the tire radial direction and along the belt layer  14  also bulge inward in the tire radial direction at a position located inward of the center land portion  21  in the tire radial direction. 
     In the pneumatic tire  1  according to the third embodiment, the belt layer  14  bulges inward in the tire radial direction internal-pressure uninflated state at the position located inward of the center land portion  21  in the tire radial direction. Thus, the tread rubber layer  4  located in the center region Ac can be more reliably provided with a large rubber thickness. Accordingly, the strength at break of the center region Ac can be more reliably increased. As a result, the shock burst resistance performance can be more reliably improved. 
     Fourth Embodiment 
     The pneumatic tire  1  according to a fourth embodiment has a configuration substantially similar to the configuration of the pneumatic tire  1  according to the first embodiment. However, the pneumatic tire  1  according to the fourth embodiment has the feature that a belt reinforcing layer  15  is disposed outward of the belt layer  14  in the tire radial direction. The other components are similar to the corresponding components of the first embodiment, and thus description of the components is omitted and the same reference numerals are used. 
       FIG. 8  is a detailed cross-sectional view of a main portion of the pneumatic tire according to a fourth embodiment. In the pneumatic tire  1  according to the fourth embodiment, in the tread portion  2 , the relationship among the tire average thickness Gc in the center region Ac, the tire average thickness Gsh in the shoulder regions Ash, and the tire average thickness Gm in the intermediate regions Am is within the range of 1.05≤(Gc/Gsh)≤1.35 and satisfies the relationship Gc≥Gm&gt;Gsh, as is the case with the pneumatic tire  1  according to the first embodiment. 
     Furthermore, in the pneumatic tire  1  according to the fourth embodiment, the belt reinforcing layer  15  is disposed outward of the belt layer  14  in the tire radial direction. The belt reinforcing layer  15  is disposed outward of the belt layer  14  in the tire radial direction to cover the belt layer  14  in the tire circumferential direction. The belt reinforcing layer  15  is formed of coating rubber-covered cords (not illustrated) disposed side by side in the tire lateral direction and substantially parallel to the tire circumferential direction. The cords of the belt reinforcing layer  15  are made of steel or organic fibers such as polyester, rayon, or nylon, and the angle of the cords is within a range of ±5° with respect to the tire circumferential direction. In the fourth embodiment, the belt reinforcing layer  15  is disposed all over the region in the tire lateral direction in which the belt layer  14  is disposed, and covers the end portions of the belt layer  14  in the tire lateral direction. 
     Additionally, the belt reinforcing layer  15 , disposed outward of the belt layer  14  in the tire radial direction, is provided by winding, in the tire circumferential direction, a band-like strip material having, for example, a width of approximately  10  mm. In other words, the belt reinforcing layer  15  is disposed by spirally winding the band-like strip material outward of the belt layer  14  in the tire radial direction. At this time, the band-like strip material is wound in one layer at a position located inward, in the tire radial direction, of the second land portions  22  and the shoulder land portions  23 , and in two layers overlapping in the tire radial direction, at a position located inward of the center land portion  21  in the tire radial direction. In other words, the band-like strip material is spirally wound without overlapping in the tire radial direction at the position located inward of the second land portions  22  and the shoulder land portions  23 , while being wound in such a manner as to overlap the strip material itself in the tire radial direction at the position located inward of the center land portion  21  in the tire radial direction. Thus, in the belt reinforcing layer  15 , a larger number of strip materials are layered at the position of the center region Ac than at the positions other than the position of the center region Ac. 
     In the pneumatic tire  1  according to the fourth embodiment, the belt reinforcing layer  15  is disposed outward of the belt layer  14  in the tire radial direction as described above, and thus inappropriate deflection of the tread portion  2  is suppressed in a case where the ground contact surface  3  comes into contact with the ground. This enables a reduction in difference in contact length between the central region in the tire lateral direction and the end portion region in the tire lateral direction which difference occurs in a case where the ground contact surface  3  contacts the ground, allowing suppression of an increase in rolling resistance caused by a significant difference in contact length depending on the position in the tire lateral direction. 
     Additionally, since a larger number of the belt reinforcing layers  15  are layered at the position of the center region Ac than at the positions other than the position of the center region Ac, the strength at break of the center region Ac can be more reliably increased. Damage caused to the belt layer  14  by the projection  105  can be more reliably suppressed in a case where the projection  105  is treaded by the tread portion  2 . As a result, both shock burst resistance performance and reduced rolling resistance can be more reliably provided in a compatible manner. 
     Fifth Embodiment 
     The pneumatic tire  1  according to a fifth embodiment has a configuration substantially similar to the configuration of the pneumatic tire  1  according to the first embodiment. However, the pneumatic tire  1  according to the fifth embodiment has the feature that a circumferential narrow groove  45  is formed in the shoulder regions Ash. The other components are similar to the corresponding components of the first embodiment, and thus description of the components is omitted and the same reference numerals are used. 
       FIG. 9  is a detailed cross-sectional view of a main portion of the pneumatic tire  1  according to the fifth embodiment. In the pneumatic tire  1  according to the fifth embodiment, in the tread portion  2 , the relationship among the tire average thickness Gc in the center region Ac, the tire average thickness Gsh in the shoulder regions Ash, and the tire average thickness Gm in the intermediate regions Am is within the range of 1.05≤(Gc/Gsh)≤1.35 and satisfies Gc≥Gm&gt;Gsh, as is the case with the pneumatic tire  1  according to the first embodiment. 
     Additionally, in the pneumatic tire  1  according to the fifth embodiment, the circumferential narrow groove  45  extending in the tire circumferential direction is formed in at least one of the shoulder land portions  23  located on the opposite sides in the tire lateral direction. Specifically, the circumferential narrow groove  45  is disposed in the shoulder region Ash in the shoulder land portion  23 . In other words, the circumferential narrow groove  45  is formed in at least one of the shoulder regions Ash located on the opposite sides in the tire lateral direction. The circumferential narrow groove  45  formed in the shoulder region Ash includes an opening portion extending to the ground contact surface  3  and having a groove width of 0.6 mm or more and 2 mm or less and a groove depth of 3 mm or more and 5 mm or less. 
     Furthermore, in the pneumatic tire  1  according to the fifth embodiment, portions of the carcass layer  13  located in the shoulder regions Ash bulge toward the tire inner surface  18  in the internal-pressure uninflated state. In other words, in the carcass layer  13  in the internal-pressure uninflated state, the most of a portion of the carcass layer  13  located in the tread portion  2  bulges outward in the tire radial direction, and the most of a portion of the carcass layer  13  located in the sidewall portion  8  bulges outward in the tire lateral direction. Specifically, in the carcass layer  13 , most portions of the carcass layer  13  other than the bead portion  10  bulge toward the tire outer side surface in the internal-pressure uninflated state for the pneumatic tire  1 , whereas portions of the carcass layer  13  located in the shoulder regions Ash are formed to bulge toward the tire inner surface  18 . The carcass layer  13  includes inward bulging portions  13   a  in portions of the layer  13  located at the shoulder regions Ash, the inward bulging portions  13   a  bulging toward the tire inner surface  18  as described above in the internal-pressure uninflated state. 
     In a case where the pneumatic tire  1  according to the fifth embodiment is used, as is the case with the pneumatic tire  1  according to the first embodiment, the pneumatic tire  1  is mounted on the rim wheel R by fitting the rim wheel R to the bead portion  10 , and is then inflated by filling the inside with air. In a case where the inside is filled with air, tension is applied to the whole pneumatic tire  1  due to the internal pressure, and the tension is mainly received by the carcass layer  13 . In other words, the carcass layer  13  extends between the pair of bead portions  10  and has a role as the backbone of the pneumatic tire  1 . Thus, tension caused by the internal pressure is mainly received by the carcass layer  13 . Thus, in an internal-pressure inflated state, a high tensile force acts on the carcass layer  13 . 
     In contrast, the carcass layer  13  included in the pneumatic tire  1  according to the fifth embodiment include the inward bulging portions  13   a  in the portions of the layer  13  located at the shoulder regions Ash. After the pneumatic tire  1  is inflated to an internal pressure to cause tension to act on the carcass layer  13 , the inward bulging portion  13   a  of the carcass layer  13  is shaped to bulge toward the tire outer side surface due to the internal pressure. Subsequently, tension acts on the inward bulging portion  13   a.  Thus, even after the internal-pressure inflation, the inward bulging portion  13   a  of the carcass layer  13  can keep the tension acting due to the internal pressure low, enabling a reduction in bending rigidity at or near the shoulder regions Ash. Accordingly, in a case where the projection  105  is treaded by the vicinity of the center region Ac of the tread portion  2 , the shoulder regions Ash can be more reliably preferentially deformed, enabling a reduction in pressure applied to the tread portion  2  by the projection  105 . As a result, the shock burst resistance performance can be more reliably improved. 
     Additionally, in the pneumatic tire  1  according to the fifth embodiment, since the circumferential narrow groove  45  is formed in the shoulder region Ash, the rigidity of the shoulder region Ash in the shoulder land portion  23  can be reduced. Thus, in a case where a load acts on the shoulder region Ash, possible strain can be relieved. This enables a more reliable reduction in energy loss in the shoulder regions Ash where, while the vehicle is traveling, the energy loss during rotation of the pneumatic tire  1  is likely to increase. Accordingly, the resistance offered in a case where the shoulder regions Ash deflect can be more reliably reduced, and thus the energy loss during rotation of the pneumatic tire  1  can be reduced, enabling a reduction in rolling resistance. 
     Additionally, the circumferential narrow groove  45  formed in the shoulder region Ash allow the shoulder region Ash to deform preferentially in a case where the projection  105  is treaded by the vicinity of the center region Ac of the tread portion  2 , allowing the whole tread portion  2  to deflect. This enables a reduction in pressure applied to the tread portion  2  by the projection  105 . Accordingly, a possible shock burst can be suppressed. As a result, both shock burst resistance performance and reduced rolling resistance can be more reliably provided in a compatible manner. 
     Sixth Embodiment 
     The pneumatic tire  1  according to a sixth embodiment has a configuration substantially similar to the configuration of the pneumatic tire  1  according to the first embodiment. However, the pneumatic tire  1  according to the sixth embodiment includes side reinforcing rubber  50  in the sidewall portions  8 . The other components are similar to the corresponding components of the first embodiment, and thus description of the components is omitted and the same reference numerals are used. 
       FIG. 10  is a detailed cross-sectional view illustrating a main portion of the pneumatic tire  1  according to the sixth embodiment. In the pneumatic tire  1  according to the sixth embodiment, in the tread portion  2 , the relationship among the tire average thickness Gc in the center region Ac, the tire average thickness Gsh in the shoulder regions Ash, and the tire average thickness Gm in the intermediate regions Am is within the range of 1.05≤(Gc/Gsh)≤1.35 and satisfies the relationship Gc≥Gm&gt;Gsh, as is the case with the pneumatic tire  1  according to the first embodiment. 
     Additionally, the pneumatic tire  1  according to the sixth embodiment includes the side reinforcing rubber  50  in the sidewall portions  8 , and is used as what is called a run-flat tire that can travel even in a case where air escapes due to a puncture or the like. The side reinforcing rubber  50  disposed in the sidewall portions  8  is a rubber member provided inside the sidewall portions  8 , and is disposed without being exposed from the tire inner surface or the tire outer surface. Specifically, the side reinforcing rubber  50  is located mainly inward, in the tire lateral direction, of portions of the carcass layer  13  located at the sidewall portions  8 . In each sidewall portion  8 , the side reinforcing rubber  50  is disposed between the carcass layer  13  and the innerliner  16 , and is shaped like a crescent such that the shape of the side reinforcing rubber  50  in a meridian cross-section of the pneumatic tire  1  projects outward in the tire lateral direction. 
     The side reinforcing rubber  50  shaped like a crescent includes an outer end portion  51  corresponding to an outer end portion in the tire radial direction and located inward, in the tire radial direction, of the belt layer  14  in the tread portion  2 , and the side reinforcing rubber  50  and the belt layer  14  are disposed partially overlapping with each other in the tire radial direction by a predetermined range of amount of overlap. Thus, at least a portion of the side reinforcing rubber  50  near the outer end portion  51  is located in the shoulder region Ash. The side reinforcing rubber  50  thus disposed is formed of a rubber material having a higher strength than the rubber forming the sidewall portions  8  and the rim cushion rubber  17  disposed in the bead portions  10 . 
     The side reinforcing rubber  50  may be partially located in the intermediate region Am rather than being located only in the shoulder region Ash. Additionally, in a case where a portion of the side reinforcing rubber  50  is located in the shoulder region Ash or the intermediate region Am, the tire average thickness Gsh in the shoulder regions Ash and the tire average thickness Gm in the intermediate regions Am include the thickness of the side reinforcing rubber  50 . 
     In the pneumatic tire  1  according to the sixth embodiment, the side reinforcing rubber  50  is disposed inward of the sidewall portion  8  as described above, thus increasing the bending rigidity of the sidewall portion  8 . Thus, even in a case where air escapes due to a puncture or the like to cause a heavy load to act on the sidewall portion  8 , deformation of the sidewall portion  8  can be reduced, and traveling can be performed as long as the speed has a predetermined value or smaller. 
     On the other hand, in the run-flat tire, the side reinforcing rubber  50  is disposed in the sidewall portion  8  to increase the bending rigidity of the sidewall portion  8 . Thus, in a case where the projection  105  is treaded in the internal-pressure inflated state, the sidewall portion  8  is less likely to deflect. Thus, stress generated in a case where the projection  105  is treaded is likely to concentrate at the tread portion  2 , making a shock burst more likely to occur. 
     In contrast, the pneumatic tire  1  according to the sixth embodiment has an increased tire average thickness Gc in the center region Ac and a reduced tire average thickness Gsh in the shoulder regions Ash. Thus, in a case where the projection  105  is treaded by the tread portion  2 , the shoulder regions Ash are more likely to deform. Accordingly, the pressure applied to the tread portion  2  by the projection  105  can be reduced in a case where the projection  105  is treaded, allowing suppression of penetration of the tread portion  2  by the projection  105 , which leads to a shock burst. As a result, both run-flat performance and shock burst resistance performance can be provided in a compatible manner. 
     Modified Examples 
     Note that, in the above-described first embodiment, four main grooves  30  are formed but that the number of the main grooves  30  may be different from four. Additionally, in the above-described first embodiment, the center region Ac coincides with the range, in the tire lateral direction, of the center  1  and portion  21  corresponding to the land portion  20  located on the tire equatorial plane CL. However, the center region Ac need not be located on the tire equatorial plane CL. For example, in a case where the main groove  30  is located on the tire equatorial plane CL, the center region Ac may correspond to a range, in the tire lateral direction, of the land portion  20  which range is defined by the main groove  30  located on the tire equatorial plane CL and the main groove  30  second closest to the tire equatorial plane CL. In other words, it is sufficient that a portion of the region interposed between two adjacent main grooves  30  which portion is closest to the tire equatorial plane CL is used as the center region Ac. 
     Additionally, in a case where the position of center of the center region Ac in the tire lateral direction differs from the position of the tire equatorial plane CL in the tire lateral direction, the intermediate regions Am located on the opposite sides of the tire equatorial plane CL in the tire lateral direction may have different widths in the tire lateral direction. In this case, the tire average thickness Gm in the intermediate regions Am has an average value for the intermediate regions Am located on the opposite sides of the tire equatorial plane CL in the tire lateral direction. 
     Additionally, in the first embodiment described above, the lug grooves  40  are not formed across the adjacent main grooves  30 , but the lug grooves  40  may be formed across the adjacent main grooves  30 . In other words, the land portion  20  in each region may be formed in a rib shape extending in the tire lateral direction, or in a block shape defined by the main grooves  30  adjacent to the land portion  20  in the tire lateral direction and the lug grooves  40  adjacent to the land portion  20  in the tire circumferential direction. 
     Additionally, in the above-described first embodiment, the rubber of the tread rubber layer  4  located in the center region Ac is 10 MPa or more and 16 MPa or less in modulus of 300% elongation. However, for the rubber forming the tread rubber layer  4 , the rubber located in the regions other than the center region Ac may also be 10 MPa or more and 16 MPa or less in modulus of 300% elongation. 
     Additionally, in the second embodiment described above, the center land portion  21  corresponds to the protrusion-shaped land portion  26 . However, any of the land portions  20  other than the center land portion  21  may be formed as the protrusion-shaped land portion  26 . For example, the second land portions  22  located in the intermediate regions Am may be formed as the protrusion-shaped land portions  26 , or both the center land portion  21  located in the center region Ac and each of the second land portions  22  positioned in the intermediate regions Am may be formed as the protrusion-shaped land portions  26 . In the tread portion  2 , at least one of the land portions  20  located in the center region Ac and the intermediate regions Am may be formed as the protrusion-shaped land portion  26 . 
     Additionally, in the third embodiment described above, the belt layer  14  bulges inward in the tire radial direction at the position located inward of the center land portion  21  in the tire radial direction. However, the belt layer  14  may bulge inward in the tire radial direction at a position located inward, in the tire radial direction, of any portion other than the center land portion  21 . For example, the belt layer  14  may bulge inward in the tire radial direction at a position located inward, in the tire radial direction, of the second land portion  22  located in each intermediate region Am, or may bulge inward in the tire radial direction at positions located inward, in the tire radial direction, of both the center land portion  21  positioned in the center region Ac and the second land portion  22  positioned in each intermediate region Am. It is sufficient that the belt layer  14  bulges inward in the tire radial direction at the position located inward, in the tire radial direction, of at least one land portion  20  of the land portions  20  located in the center region Ac and the intermediate regions Am. 
     Additionally, in the fifth embodiment described above, the circumferential narrow groove  45  formed in the shoulder region Ash may be formed in the shoulder regions Ash on the opposite sides in the tire lateral direction or may be formed in one of the shoulder regions Ash. Additionally, the circumferential narrow groove  45  need not necessarily be formed continuously all around the circumference, and the circumferential narrow groove  45  may be discontinuous in the tire circumferential direction to the extent that the function of the circumferential narrow groove  45  to reduce the rigidity of the shoulder regions Ash is not degraded. 
     Additionally, in the fifth embodiment described above, the carcass layer  13  includes the inward bulging portion  13   a  in the portion located in each shoulder region Ash. However, the inward bulging portion  13   a  need not clearly bulge toward the tire inner surface  18  in the internal-pressure uninflated state. For example, the inward bulging portion  13   a  may be formed in a linear shape or in a wavy shape in a tire meridian cross-sectional view in the internal-pressure uninflated state. The inward bulging portion  13   a  of the carcass layer  13  is shaped to bulge toward the tire outer side surface due to the tension acting on the carcass layer  13  in the internal-pressure inflated state. However, at this time, the inward bulging portion  13   a  may have any shape as long as the shape enables a reduction in tension in the portion of the carcass layer  13  located in the shoulder region Ash. 
     Additionally, the above-described first to sixth embodiments and modified examples may be appropriately combined. For example, any of the configurations illustrated in the second to fifth embodiments may be combined with the side reinforcing rubber  50  illustrated in the sixth embodiment. In the pneumatic tire  1 , in a case where, in at least the tread portion  2 , the relationship among the tire average thickness Gc in the center region Ac, the tire average thickness Gsh in the shoulder regions Ash, and the tire average thickness Gm in the intermediate regions Am is within the range of 1.05≤(Gc/Gsh)≤1.35 and satisfies the relationship Gc≥Gm&gt;Gsh, both shock burst resistance performance and reduced rolling resistance can be provided in a compatible manner. 
     EXAMPLES 
       FIGS. 11A to 11C  are tables showing the results of performance tests of pneumatic tires. In relation to the pneumatic tire  1  described above, performance evaluation tests will be described below that were conducted on a pneumatic tire according to a conventional example, the pneumatic tires  1  according to present technology, and a pneumatic tire according to a comparative example for comparison with the pneumatic tire  1  according to the present technology. The performance evaluation tests were performed on the shock burst resistance, indicating durability against shock bursts, and rolling resistance performance indicating performance related to the rolling resistance. 
     The performance evaluation tests were performed using the pneumatic tire  1  having a nominal size of 245/50R19 105W, specified by JATMA, and the pneumatic tire  1  was mounted on a JATMA standard rim wheel with a rim size of 19×7.5J. In the evaluation method for each of the test items, for the shock burst resistance performance, the test tires were inflated to an air pressure of 220 kPa, and plunger destructive tests were conducted at a plunger diameter of 19 mm and at an insertional speed of 50 mm/min accordance with JIS K6302 to measure tire fracture energy. The shock burst resistance is expressed as index values with the conventional example described below being assigned the value of 100. Larger index values indicate higher tire strength and more excellent shock burst resistance. 
     Additionally, for the rolling resistance performance, the test tires are inflated to 250 kPa, and the rolling resistance was measured after 30 min pre-running at a drum radius of 854 mm, at a speed of 80 km/h, and at a load of 7.26 kN. The rolling resistance performance is expressed as index values each corresponding to the reciprocal of the measured rolling resistance, with the conventional example described below being assigned the value of 100. Larger index values indicate lower rolling resistance. 
     The performance evaluation tests were conducted on 21 types of pneumatic tires including the pneumatic tire according to Conventional Example corresponding to an example of conventional pneumatic tire, Examples 1 to 17 corresponding to the pneumatic tires  1  according to the present technology, and Comparative Examples 1 to 3 corresponding to pneumatic tires compared with the pneumatic tire  1  according to the present technology. In the pneumatic tire according to Conventional Example, the tire average thickness Gc in the center region Ac of the tread portion  2  is smaller than the tire average thickness Gsh in the shoulder regions Ash. Additionally, in the pneumatic tires according to Comparative Examples 1 to 3, the relationship among the tire average thickness Gc in the center region Ac, the tire average thickness Gsh in the shoulder regions Ash, and the tire average thickness Gm in the intermediate regions Am is not within the range of 1.05≤(Gc/Gsh)≤1.35 or does not satisfy Gc≥Gm&gt;Gsh. 
     In contrast, in all of Examples 1 to 17 corresponding to examples of the pneumatic tire  1  according to the present technology, the tire average thickness Gc in the center region Ac with respect to the tire average thickness Gsh in the shoulder regions Ash is within the range of 1.05≤(Gc/Gsh)≤1.35, and the tire average thickness Gc in the center region Ac, the tire average thickness Gsh in the shoulder regions Ash, and the tire average thickness Gm in the intermediate regions Am satisfy the relationship Gc≥Gm&gt;Gsh. Furthermore, the pneumatic tires  1  according to Examples 1 to 17 differ in the average thickness Tc of the tread rubber layer  4  in the center region Ac with respect to the average thickness Tsh of the tread rubber layer  4  in the shoulder regions Ash (Tc/Tsh), the average thickness Tm of the tread rubber layer  4  in the intermediate regions Am with respect to the average thickness Tsh of the tread rubber layer  4  in the shoulder regions Ash (Tm/Tsh), the average actual rubber thickness Vc of the tread rubber layer  4  in the center region Ac with respect to the average actual rubber thickness Vsh of the tread rubber layer  4  in the shoulder regions Ash (Vc/Vsh), the presence of the protrusion-shaped land portion  26 , the radius of curvature RR of the ground contact surface  3  of the protrusion-shaped land portion  26  with respect to the radius of curvature TR of the tread profile PR (RR/TR), the presence of a portion of the belt layer  14  bulging inward in the tire radial direction, the minimum thickness Tg of the rubber thickness between the groove bottom  36  of the center main groove  31  and the belt layer  14  with respect to the average thickness Tc of the tread rubber layer  4  in the center region Ac (Tg/Tc), the modulus MPa of 300% elongation of the tread rubber layer  4  in the center region Ac, the presence of the circumferential narrow groove  45  in the shoulder regions Ash, the presence of the inward bulging portion  13   a  of the carcass layer  13 , the presence of the belt reinforcing layer  15 , whether a large number of pieces of the belt reinforcing layer  15  are layered in the center region Ac, and the presence of the side reinforcing rubber  50 . 
     The results of the performance evaluation tests using these pneumatic tires  1  indicate that the pneumatic tires  1  according to Examples 1 to 17 can improve both the shock burst resistance and the rolling resistance compared to Conventional Example without degrading either of the performances compared to Conventional Example, as illustrated in  FIGS. 11A to 11C . In other words, the pneumatic tires  1  according to Examples 1 to 17 can provide both shock burst resistance performance and reduced rolling resistance in a compatible manner.