_______________________________________________________________________________

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

U.S. Department

of Transportation

National Highway DOT HS 809 359

Traffic Safety

Administration October 2001

Tire Pressure Special Study

Tread Depth Analysis

Kristin Thiriez and Rajesh Subramanian*

 

* Kristin Thiriez is a General Engineer in the Crash Investigation Division and Rajesh Subramanian is a Programmer 

Analyst in the Mathematical Analysis Division in NHTSA’s National Center for Statistics and Analysis. 

Nine percent of vehicles had at least one 

tire that was bald, with a measured tread 

depth of 2/32nds of an inch or less.

“Tire Pressure Special Study: Tread Depth 

Analysis” is the fourth in a series of research 

notes containing results from the Tire

Pressure Special Study (TPSS) conducted in 

2001 by the National Center for Statistics and 

Analysis of the National Highway Traffic 

Safety Administration (NHTSA). The focus 

of this research note is on results from the tire 

measurements. Previous topics of TPSS 

research notes included the study 

methodology, the results from the driver 

interviews, and the vehicle observation data.

Background

In 2000, Congress passed the Transportation 

Recall Enhancement, Accountability, and 

Documentation (TREAD) Act. Section 13 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 13 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 monitoring system. 

Data Collection Methodology

Field data collection was conducted by NCSA 

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 

Research Note
_______________________________________________________________________________

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

vehicles was consistent with national 

estimates of vehicle registration.

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. 

The researchers were given forms on which to 

record the vehicle and tire measurements, as 

well as detailed instructions on how to 

proceed in order to promote consistency. The 

following was excerpted from the instruction 

document: 

“The Measured Minimum Tire Tread Depth is 

to be measured using the supplied tire tread 

depth indicator. The measurement should be 

taken on the shallowest groove of the tread. 

Be careful not to measure on a wear bar 

indicator. The measurement is to be 

documented to the nearest thirty-second of an 

inch (X/32”).

Image 1: Procedure for Tread Depth Measurement: “on the 

shallowest groove.”

“The tire tread depth indicator is read by 

placing the indicator over the top of a groove 

and pressing down on the measurement rod 

until contact is made with the bottom of the 

groove. Read the indicator by rotating it until 

a line on the measuerment rod is flush with 

the base.”

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

Tread depth data were analyzed only for 

passenger cars with P-metric tires to reduce 

variation in initial tread depths. P-metric tires 

are regular passenger car tires and the labeling 

has the format “P205/75R14.” Initial tread 

depths can vary from 8-11/32nds of an inch, 

while light truck tires have a much wider 

range. 

Included in this analysis were the 6,240 

vehicles that were passenger cars with Pmetric tires and had a measured tread depth 

for each of the vehicle’s four tires.

The data were analyzed by first creating a 

frequency distribution of the measured tread 

depths. Then, average tread depths were 

calculated for each tire and for each vehicle 

and those distributions graphed. Minimum 

tread depths were also determined for each 

vehicle and then for each axle and graphed.

For the purpose of this anaylsis, tires with 

2/32nds of an inch or less tread depth were 

considered “bald” tires.

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. 

Results of the Tire Measurements 

Figure 1 shows the frequency distribution of 

the measured tread depth for each tire. 

Figure 2 shows the cumulative percent of the 

measured pressure at each tire.
________________________________________________________________________

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

Figure 1

 Percent Distribution of the Measured Tread Depth for Each Tire

0

5

10

15

20

25

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Measured Tread Depth (in 32nds of an inch)

Percent

Left Front Tire Right Front Tire Left Rear Tire Right Rear Tire

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

Figure 2

Cumulative Percent of the Measured Tread Depth for Each Tire

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Measured Tread Depth (in 32nd of an inch)

Cumulative Percent

Left Front Tire Right Front Tire Left Rear Tire Right Rear Tire

TD<2/32" =

 BALD TIRE

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

TD<2/32” =

BALD TIRE
________________________________________________________________________

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

Table 1 shows that the average tread 

depth for each tire was around 7/32nds 

of an inch. As can be seen from Figure 

2, above, and from Table 2, below, for 

each tire position except the left rear, 

almost 4% of vehicles had a bald tire at 

that position. Only 3% of vehicles had a 

bald tire in the left rear tire position.

Table 1

Average Tread Depth for Each Tire

(Estimates in Thirty-seconds of an Inch)

Left Front Right Front Left Rear Right Rear 

Tread

6.8 6.8 7.0 6.8

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

The average tread depth for each vehicle 

was calculated. The results are shown in 

Figures 3 and 4. Figure 3 shows the 

frequency distribution of the average of 

the measured tread depths of the four 

tires for each vehicle. 

Figure 4 shows the percent distribution 

of the average tread depth for each 

vehicle. Table 3 shows that less than 

1.5% of vehicles had an average tread 

depth of 2/32nds of an inch or below.

Table 2

Percent Distribution of the Measured Tread Depth for Each Tire

Tread Left Front Right Front Left Rear Right Rear

(X/32”) % Cum% % Cum% % Cum% % Cum%

0 0.8 0.8 0.6 0.6 0.6 0.6 0.7 0.7

1 1.3 2.0 1.3 1.9 0.6 1.1 1.0 1.7

2 1.5 3.5 1.9 3.8 1.4 2.5 1.9 3.7

3 3.7 7.2 4.1 8.0 3.7 6.2 4.7 8.4

4 9.3 16.5 9.9 17.8 10.0 16.3 11.0 19.3

5 10.7 27.2 10.0 27.8 10.2 26.4 9.6 28.9

6 12.0 39.2 12.7 40.5 10.8 37.3 10.5 39.4

7 18.7 57.9 18.1 58.7 15.7 53.0 16.8 56.3

8 21.0 78.9 20.5 79.2 22.0 75.0 19.3 75.5

9 10.7 89.6 9.8 89.0 12.6 87.6 11.9 87.4

10 6.7 96.3 7.6 96.5 8.1 95.7 8.1 95.5

11 3.1 99.3 2.7 99.2 3.7 99.4 3.8 99.3

12 0.5 99.8 0.6 99.8 0.5 99.9 0.6 99.9

13 0.2 100.0 0.2 100.0 0.0 99.9 0.0 100.0

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 

Percent Distribution

 of the Average Measured Tread Depth for Each Vehicle

0

5

10

15

20

25

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Average Measured Tread Depth (in 32nds of an inch)

Percent

All Tires

TD<2/32" = 

BALD TIRE

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

Figure 4 

Cumulative Percent 

of the Average Measured Tread Depth for Each Vehicle

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Average Measured Tread Depth (in 32nds of an inch)

Cumulative Percent

All Tires

TD<2/32" = 

BALD TIRE

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 3

Percent of the Average Measured Tread Depth for Each Vehicle

Average Tread (X/32”) Percent Cumulative Percent

0 0.0 0.0

1 0.2 0.2

2 0.9 1.2

3 3.4 4.6

4 6.9 11.4

5 10.7 22.1

6 16.3 38.4

7 19.7 58.1

8 20.3 78.5

9 12.4 90.9

10 6.5 97.4

11 2.3 99.7

12 0.3 100.0

13 0.0 100.0

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

For each vehicle, the tire with the lowest 

measured tread depth was determined. 

Figure 5 shows the frequency 

distribution of the minimum measured 

tread depth for each vehicle.

Figure 5 

Percent Distribution 

of the Minimum Measured Tread Depth for Each Vehicle

0

5

10

15

20

25

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Minimum Measured Tread Depth (in 32nds of an inch)

Percent

All Tires

TD<2/32" = 

BALD TIRE

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 6 and Table 4, below, show that 

almost 9% of vehicles had at least one 

tire that was bald, with a measured 

tread depth of 2/32nds of an inch or less.

Figure 6 

Cumulative Percent 

of the Minimum Measured Tread Depth for Each Vehicle

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Minimum Measured Tread Depth (in 32nds of an inch)

Cumulative Percent

All Tires

TD<2/32" = 

BALD TIRE

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

Table 4

Percent of the Minimum Measured Tread Depth for Each Vehicle

Minimum 

Tread

Both 

Axles% Cum% Front Axle% Cum% Rear Axle% Cum%

0 2.2 2.2 1.1 1.1 1.0 1.0

1 2.7 4.9 1.9 3.0 1.4 2.4

2 3.8 8.7 2.3 5.3 2.4 4.9

3 8.2 16.9 5.4 10.7 6.0 10.8

4 16.7 33.6 12.3 23.0 13.0 23.9

5 13.7 47.3 11.7 34.7 11.0 34.9

6 13.3 60.7 13.4 48.1 11.7 46.6

7 15.2 75.8 17.5 65.6 16.3 62.8

8 14.2 90.0 18.3 83.9 18.2 81.1

9 5.9 95.9 8.8 92.7 10.0 91.1

10 3.2 99.1 5.3 98.0 6.6 97.7

11 0.7 99.8 1.6 99.6 1.9 99.6

12 0.1 100.0 0.2 99.8 0.4 100.0

13 0.0 100.0 0.2 100.0 0.0 100.0

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 7 shows the frequency 

distribution of the minimum measured 

tread depth for the front axle and the rear 

axle of each vehicle.

Figure 7 

Percent Distribution of the Minimum Measured Tread Depth of Each 

Axle for Each Vehicle

0

5

10

15

20

25

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Minimum Measured Tread Depth (in 32nds of an inch)

Percent

Front Axle Rear Axle

TD<2/32" = 

BALD TIRE

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

Figure 8

Cumulative Percent of the Minimum Measured Tread Depth 

of Each Axle for Each Vehicle

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Minimum Measured Tread Depth (in 32nds of an inch)

Cumulative Percent

Front Axle Rear Axle

TD<2/32" = 

BALD TIRE

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

Analysis of Tire Pressure Relative to 

Tread Depth

Tread Depth was analyzed for its 

relationship with under-inflation. 

Unweighted data were used in this 

analysis; thus the percentages do not 

necessarily represent national estimates. 

The sample size of over 6,000 vehicles 

was sufficiently large to show a 

correlation between tread depth and 

under-inflation. The results are shown 

in Table 5. 

Table 5 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 tread 

depth of the tire. Categories were 

broken down by tread depth and extent 

of under-inflation. With under-inflation, 

there was a correlation between the tread 

depth of the tire and the amount of 

deviation in pressure, with balder tires 

being more likely to be under-inflated 

than tires with deeper tread.

Table 5

Percentage of Tires Deviating from the Manufacturer’s Recommended Pressure (Delta P)

by Tire Position, Tread Depth and Delta P (D) (Under-inflation Only)

Tread Depth (X/32”)

Tread Depth £ 2 2 < Tread Depth £ 4 Tread Depth > 4

Tire 

Position

Delta P in psi Delta P in psi Delta P in psi

D 3 8 D < 8 D 3 8 D < 8 D 3 8 D < 8

LF 13.4 86.6 10.8 89.2 8.5 91.5

RF 15.8 84.2 15.3 84.7 8.8 91.2

LR 22.2 77.8 19.4 80.6 14.9 85.1

RR 25.6 74.4 19.5 80.5 15.3 84.7

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

Figure 9 is a graphical representation of Table 

5 and shows that for vehicles that have tires 

with shallower tread depths, the percentage of 

those tires also being under-inflated by 8psi or 

more was higher than for vehicles that have 

tires with deeper tread depths. The figure also 

shows that rear tires with any kind of tread 

depth were more likely to be under-inflated 

than front tires.
Figure 9 

For the Given Ranges of Tread Depth, 

the Percentage of Tires Underinflated by 8psi or More (by Tire)

0

5

10

15

20

25

30

TD<=2 2<TD<=4 TD>4

Tread Depth (in 32nds of an inch)

Percentage

Left Front Tire Right Front Tire Left Rear Tire Right Rear Tire

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

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 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