_______________________________________________________________________________ 

 National Center for Statistics and Analysis 400 Seventh St., S.W., Washington, DC 20590 

U.S. Department 

of Transportation 

National Highway DOT HS 809 317 

Traffic Safety 

Administration August 2001

Tire Pressure Special Study 

Vehicle Observation Data 

Twenty seven percent of passenger cars with 

regular passenger car tires have at least one 

tire that is underinflated by 8 psi or more. 

“Tire Pressure Special Study: Vehicle 

Observation Data” is the third in a series of 

research notes containing results from the 

Tire Pressure Special Study (TPSS) 

conducted by the National Highway Traffic 

Safety Administration (NHTSA) in 2001. 

The focus of this research note is on results 

from the vehicle inspections and tire 

measurements. Previous topics of TPSS 

research notes included the study 

methodology and the results from the driver 

interview data. 

Background 

In 2000, Congress passed the Transportation 

Recall Enhancement, Accountability, and 

Documentation (TREAD) Act. Section 12 of 

this act directed the Department of 

Transportation to complete a rulemaking 

within one year, requiring implementation of 

a warning system in new motor vehicles 

indicating significantly under-inflated tires. 

In response to Section 12 of the TREAD Act, 

NHTSA’s National Center for Statistics and 

Analysis (NCSA) conducted the TPSS. The 

TPSS was designed to assess to what extent 

passenger vehicle operators are aware of the 

recommended tire pressures for their vehicles, 

the frequency and the means they use to 

measure their tire pressure, and how 

significantly actual measured tire pressure 

differed from the manufacturer’s 

recommended tire pressure. The data 

collected are being used to support various 

rulemaking actions including an upgrade to 

the placement and contents of the vehicle 

placard and the development of an onboard 

tire pressure measuring sensor. (These data 

were presented at a public meeting on July 26, 

2001, and were placed in Docket Number 

NHTSA-2000-8572, “Tire Pressure 

Monitoring System”.) 

Data Collection Methodology 

Field data collection was conducted through 

the infrastructure of the National Automotive 

Sampling System Crashworthiness Data 

System (NASS CDS). The NASS CDS 

consists of teams of researchers located at 

Primary Sampling Units (PSUs) throughout 

the United States. The PSUs are located in 

urban, suburban, and rural settings with 

nationally representative populations. 

The population surveyed by the researchers in 

the TPSS represents a sample frame 

consisting of drivers who used gas stations to 

fill up their vehicles between the hours of 

8:00 am and 5:00 pm. Data collection was 

conducted from February 1, 2001 through 

February 14, 2001. 

Vehicles surveyed included passenger cars 

and light trucks. NHTSA classifies light 

trucks as sport utility vehicles, pickup trucks 

and vans with a Gross Vehicle Weight Rating 

of less than 10,000 pounds. A total of 11,530 

vehicles were included in the survey, of 

which 6,442 were passenger cars, 1,874 were 

sport utility vehicles, 1,376 were vans, and 

1,838 were pickup trucks. The distribution of 

vehicles was consistent with national 

estimates of vehicle registration. 

Research Note
_______________________________________________________________________________ 

 National Center for Statistics and Analysis 400 Seventh St., S.W., Washington, DC 20590 

Data collected during the TPSS included daily 

site information, driver interview and profile 

data, vehicle profile data, and tire data for all 

four tires on the vehicle. The vehicle 

information collected included vehicle profile 

data and the manufacturer’s recommended 

tire pressures. Tire information collected 

included tire profile data as well as air 

pressure, sidewall temperature and tread 

depth measurements. A complete description 

of the data collection process was presented in 

the previous research note “Tire Pressure 

Special Study: Methodology.” 

Analysis of the Vehicle Observations 

Survey data were analyzed for the following 

three categories of vehicles: 

1) Passenger Cars with P-Metric 

Tires (Cars w/ P Tires); 

2) Pickup Trucks, Sport Utility 

Vehicles (SUVs), and Vans with 

P-Metric Tires (Light Trucks w/ P 

Tires); 

3) Pickup Trucks, SUVs, and Vans 

with Other Type Tires (Light 

Trucks w/ Other Tires). 

P-Metric tires are regular passenger car tires 

and the labeling has the format 

“P205/75R14.” Other Type tires include LT 

tires, which are light truck tires with the 

format “LT235/85R15/D,” and High Flotation 

tires that have the format 

“31X10.50R15LT/C.” Information on 

standard tire labeling formats can be found in 

the “2000 Tire Guide” courtesy of the Rubber 

Manufacurers Association. 

Estimates and Sampling Error 

The observations were weighted to represent 

national estimates. Because estimates from 

the TPSS are based on a sample, they are 

statistically weighted according to the sample 

design and are subject to sampling error. 

When calculated, estimates in the tables are 

shown with their corresponding sampling 

error in parentheses. Adding and subtracting 

twice the sampling error from the 

corresponding estimate will produce an 

approximate 95 percent confidence interval 

for the estimate. This means that one can be 

95 percent confident that the true value for the 

quantity being estimated lies within this 

interval. 

Results of the Vehicle Observations 

Figures 1 through 3 show the cumulative 

percentage of the measured pressure for each 

tire for each category of vehicles analyzed. 

The corresponding tables show the measured 

pressures for given percentiles as well as the 

means and standard errors for the vehicle 

categories with P-Metric tires. 

Table 1 shows that for passenger cars with PMetric tires the average front tire pressure 

was approximately 31 psi and the average rear 

tire pressure was approximately 29 psi. The 

fifth percentile for left rear tires had a 

measured pressure of 18.8 psi or below and 

the fifth percentile for left front tires had a 

measured pressure of 22.3 psi or below. 

Table 2 shows that for pickup trucks, SUVs, 

and vans with P-Metric Tires the average 

front tire pressure was approximately 31.5 psi 

and the average rear tire pressure was 

approximately 31 psi. The fifth percentile for 

right rear tires had a measured pressure of 

21.9 psi or below. 

The average pressure for pickup trucks, 

SUVs, and vans with LT-Metric or High 

Flotation tires could not be calculated because 

of the small sample size (around 900 vehicles) 

and because of the limitations of the gauge 

used to read tire pressures. (The gauges used 

in the survey could not read above 60 psi.) 

However, the fifth percentile for right rear 

tires (Table 3) had a measured pressure of 

19.8 psi or below, compared to around 24 or 

25 psi for all of the other tires. 
_______________________________________________________________________________ 

 National Center for Statistics and Analysis 400 Seventh St., S.W., Washington, DC 20590 

Figure 1: Passenger Cars with P-Metric Tires

Cumulative Percent of Measured Pressure at Each Tire

0

10

20

30

40

50

60

70

80

90

100

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60

Measured Pressure (psi)

Cumulative Percent

LF Tire LR Tire RR Tire RF Tire

 Source: National Center for Statistics and Analysis, NHTSA, NASS 2001 Tire Pressure Special Study. 

Table 1 

Measured Pressure for Passenger Cars with P-Metric Tires 

by Tire Position and Percentile 

(Measured Pressures in psi) 

Percentile or Mean 

Tire Position 

5th Percentile 25th Percentile Mean 50th Percentile 75th Percentile 

Left Front 22.3 27.1 31.1 (0.2) 30.3 33.6 

Left Rear 18.8 25.5 29.2 (0.2) 29.0 32.2 

Right Rear 19.4 25.5 29.3 (0.2) 30.2 33.2 

Right Front 21.8 27.0 30.8 (0.2) 29.9 33.2 

Source: National Center for Statistics and Analysis, NHTSA, NASS 2001 Tire Pressure Special Study. 
________________________________________________________________________________ 

 National Center for Statistics and Analysis 400 Seventh St., S.W., Washington, DC 20590 

Figure 2: Pickup Trucks, SUVs, and Vans with P-Metric Tires

Cumulative Percent of Measured Pressure at Each Tire

0

10

20

30

40

50

60

70

80

90

100

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60

Measured Pressure (psi)

Cumulative Percent

LF LR RR RF

Source: National Center for Statistics and Analysis, NHTSA, NASS 2001 Tire Pressure Special Study. 

Table 2 

Measured Pressure for Pickup Trucks, SUVs, and Vans with P-Metric Tires 

by Tire Position and Percentile 

(Measured Pressures in psi)

Tire Percentile or Mean 

Position 5th Percentile 25th Percentile Mean 50th Percentile 75th Percentile 

Left Front 23.5 28.2 31.7 (0.3) 31.0 34.1 

Left Rear 22.9 27.6 31.1 (0.2) 30.3 33.7 

Right Rear 21.9 27.3 30.8 (0.2) 30.2 33.4 

Right Front 22.9 27.9 31.4 (0.3) 30.6 33.8 

Source: National Center for Statistics and Analysis, NHTSA, NASS 2001 Tire Pressure Special Study. 
________________________________________________________________________________ 

 National Center for Statistics and Analysis 400 Seventh St., S.W., Washington, DC 20590 

Figure 3: Pickup Trucks, SUVs, and Vans with LT-Metric or High Flotation Tires:

Cumulative Percent of Measured Pressure at Each Tire

0

10

20

30

40

50

60

70

80

90

100

9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59

Measured Pressure (psi)

Cumulative Percent

LF LR RR RF

Source: National Center for Statistics and Analysis, NHTSA, NASS 2001 Tire Pressure Special Study. 

Table 3 

Measured Pressure 

for Pickup Trucks, SUVs, and Vans with LT-Metric or High Flotation Tires 

by Tire Position and Percentile 

(Measured Pressures in psi)

Percentile or Mean 

Tire Position 

5th Percentile 25th Percentile 50th Percentile 75th Percentile 

Left Front 24.3 34.9 44.5 52.0 

Left Rear 24.5 36.1 44.1 54.8 

Right Rear 19.8 35.6 45.0 51.2 

Right Front 24.8 37.5 43.0 51.3 

Source: National Center for Statistics and Analysis, NHTSA, NASS 2001 Tire Pressure Special Study. 
________________________________________________________________________________ 

 National Center for Statistics and Analysis 400 Seventh St., S.W., Washington, DC 20590 

Figure 4 and Table 4 show the cumulative 

percent of the difference between the 

measured pressure for each tire on the 

vehicle and the manufacturer’s 

recommended cold tire pressure for that tire 

for passenger cars with P-Metric tires. The 

figure shows that rear tires were more 

underinflated with respect to the 

recommended cold tire pressure than front 

tires. Almost 10 percent of left front tires 

were underinflated by 8 psi or more and 16 

percent of left rear tires were underinflated 

by 8 psi or more. 

Figure 5 and Table 5 show the cumulative 

percent of the difference between the 

measured pressure for each tire on the 

vehicle and the manufacturer’s 

recommended cold tire pressure for pickup 

trucks, SUVs, and vans with P-Metric tires. 

The figure shows that rear tires were more 

underinflated than front tires. Eleven 

percent of left front tires were underinflated 

by 8 psi or more and 18 percent of left rear 

tires were underinflated by 8 psi or more. 

Figure 4: Passenger Cars with P-Metric Tires: Cumulative Percent of the Difference Between 

the Measured Pressure and the Manufacturer's Recommended Cold Tire Pressure (Delta P) 

by Tire

0

10

20

30

40

50

60

70

80

90

100

-20 -16 -12 -8 -4 0 4 8 12 16 20

Delta P (psi)

Cumulative Percent

LF LR RR RF

Source: National Center for Statistics and Analysis, NHTSA, NASS 2001 Tire Pressure Special Study. 
________________________________________________________________________________ 

 National Center for Statistics and Analysis 400 Seventh St., S.W., Washington, DC 20590 

Table 4 

Differences Between the Recommended and the Measured Pressure (Delta P), 

for Passenger Cars with P-Metric Tires 

by Tire Position and Percentile 

(Delta P in psi) 

Percentile or Mean 

Tire Position 

5th Percentile 25th Percentile Mean 50th Percentile 75th Percentile 

Left Front -9.9 -4.3 -0.2 (0.2) -0.8 2.7 

Left Rear -13.0 -5.9 -1.9 (0.2) -2.1 1.5 

Right Rear -12.5 -5.8 -1.8 (0.2) -2.0 1.6 

Right Front -10.1 -4.5 -0.6 (0.2) -0.9 2.3 

Source: National Center for Statistics and Analysis, NHTSA, NASS 2001 Tire Pressure Special Study.

Figure 5: PickupTrucks, SUVs, and Vans with P-Metric Tires: Cumulative Percent of the Difference 

Between the Measured Pressure and the Manufacturer's Recommended Cold Tire Pressure (Delta 

P) by Tire

0

10

20

30

40

50

60

70

80

90

100

-20 -16 -12 -8 -4 0 4 8 12 16 20

Delta P (psi)

Cumulative Percent

LF LR RR RF

Source: National Center for Statistics and Analysis, NHTSA, NASS 2001 Tire Pressure Special Study. 
________________________________________________________________________________ 

 National Center for Statistics and Analysis 400 Seventh St., S.W., Washington, DC 20590 

Table 5 

Differences Between the Recommended and the Measured Pressure (Delta P) 

for Pickup Trucks, SUVs, and Vans with P-Metric Tires 

by Tire Position and Percentile 

(Measured Pressures in psi) 

Percentile or Mean 

Tire Position 

5th Percentile 25th Percentile Mean 50th Percentile 75th Percentile 

Left Front -10.5 -5.3 -1.1 (0.4) -1.7 1.9 

Left Rear -12.7 -6.8 -2.8 (0.2) -3.4 0.4 

Right Rear -13.7 -7.2 -3.1 (0.2) -3.4 0.2 

Right Front -11.0 -5.6 -1.4 (0.4) -1.9 1.5 

Source: National Center for Statistics and Analysis, NHTSA, NASS 2001 Tire Pressure Special Study.

Table 6 shows the percentage of tires that 

deviated from the manufacturer’s 

recommended cold tire inflation pressure 

(Delta P) by 8 psi or more by the age of the 

vehicle. Categories were broken down by age 

of vehicle and extent of misinflation. Most 

tires were within 8 psi, but more tires were 

underinflated than overinflated. With 

underinflation, there was a correlation 

between the age of the vehicle and the amount 

of deviation in pressure, with older vehicles 

deviating more than newer vehicles.

.

Table 6 

Percentage of Tires Deviating from the Manufacturer’s Recommended Pressure 

by Vehicle Type, Delta P and Age of Vehicle 

(Estimates and Sampling Errors in Percentages) 

Delta P (psi) 

Underinflated by 8 or more Inflated Within 8 Overinflated by 8 or more Vehicle Type 

Vehicle Age (y) in Years Vehicle Age in (y) Years Vehicle Age (y) in Years 

y ≤ 3 3 < y ≤ 6 y > 6 y ≤ 3 3 < y ≤ 6 y > 6 y ≤ 3 3 < y ≤ 6 y > 6 

Cars with PMetric Tires 8 (1) 11 (2) 15 (1) 86 (1) 82 (1) 78 (1) 6 (1) 7 (1) 7 (1) 

Pickup Trucks, 

SUVs, and Vans 

with P-Metric 

Tires 

12 (1) 15 (2) 24 (2) 82 (1) 80 (3) 71 (2) 6 (1) 5 (1) 5 (1) 

Source: National Center for Statistics and Analysis, NHTSA, NASS 2001 Tire Pressure Special Study.
________________________________________________________________________________ 

 National Center for Statistics and Analysis 400 Seventh St., S.W., Washington, DC 20590 

Table 7 shows the percentage of vehicles by 

number of tires that were underinflated by 8 

psi or more from the recommended cold tire 

inflation pressure. More than one-quarter of 

the passenger cars with P-Metric Tires had at 

least one tire underinflated by 8 psi or more. 

For pickup trucks, vans, and SUVs with PMetric tires, almost one-third had at least one 

tire underflated by 8 psi or more. The 

proportion of pickup trucks, SUVs and vans 

with all four tires underinflated by 8 psi or 

more was twice that of passenger cars.

Table 7 

Percentage of Vehicles by Vehicle Type 

and Number of Tires Underinflated by 8 psi or More 

(Estimates and Sampling Errors in Percentages) 

Number of Tires Underinflated by 8 psi or More* Vehicle Type 

0 1 2 3 4 

Passenger Cars with P-Metric Tires 73 (2) 14 (1) 7 (1) 3 (0) 3 (0) 

Cumulative Percent 100 27 13 6 3 

Pickup Trucks, SUVs, and Vans with PMetric Tires 68 (2) 13 (1) 10 (1) 4 (1) 6 (1) 

Cumulative Percent 100 33 20 10 6 

Source: National Center for Statistics and Analysis, NHTSA, NASS 2001 Tire Pressure Special Study. 

*Percentages may not add to 100 percent due to rounding.

For additional copies of this research note, please call (202)366-4198 or fax your request to (202)366-3189. For questions 

regarding the data reported in this research, contact Nancy Bondy [202-366-5353] or Kristin Thiriez [202-366-2837] of the 

National Center for Statistics and Analysis. This research note and other general information on highway traffic safety 

may be accessed by internet users at http://www.nhtsa.dot.gov/people/ncsa. 
U.S. Department 

of Transportation 

National Highway 

Traffic Safety 

Administration

400 Seventh Street, S.W. , NRD-30 

Washington, D.C. 20590