Joseph Stevano is a Statistician employed by Rainbow Technologies, Inc., a contractor working for the 

Mathematical Anlaysis Division of the National Center for Statistics and Analysis.

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

 Research Note

U.S. Department

of Transportation DOT HS 809 366

National Highway

Traffic Safety 

Administration November 2001

Air Pumps at U.S. Gas Stations:

Major Findings Regarding Availability, Reliability and Fees

Joseph Stevano, M.S., Ph.D.

Tire manufacturers, auto manufacturers 

and experts in consumer safety all 

emphasize the importance of 

maintaining proper tire pressure for its 

positive effect on driver safety, fuel 

economy and tire longevity. Since the 

typical motorist owns neither an air 

pump nor a pressure measuring gauge, 

gas station air pumps and gauges play 

an important role with respect to driver 

safety and the conservation of scarce 

resources. 

Accordingly, this study quantifies and 

analyzes the major components 

associated with the gas station provision 

of air pressure services. These 

components can be divided into two 

groups: those associated with the 

provision of air --whether an air pump is 

present, whether it works and whether a 

fee is charged for its use-- and those 

associated with the measurement of air 

pressure --whether a gauge is present 

and, if so, how accurately it performs its 

task. Hence, this study has been 

divided into five parts: pump availability; 

pump functionality; gauge availability; 

gauge accuracy; and pump fees.

Background

In response to the rulemaking 

requirement of the Transportation Recall 

Enhancement, Accountability, and 

Documentation (TREAD ) Act, the 

National Highway Traffic Safety 

Administration (NHTSA)’s National 

Center for Statistics and Analysis 

(NCSA) conducted the National 

Automotive Sampling System (NASS) 

Tire Pressure Special Study (TPSS) in 

February 2001. Although the main 

purpose of the TPSS was to quantify the 

prevalence and magnitude of tire underinflation, information regarding air pump 

availability, functionality and whether a 

fee was charged for pump use was also 

collected from the gas stations in the 

sample. This information was 

supplemented by the NASS Air Pump 

Gauge Accuracy Special Study 

(APGAS) which went back (August

2001) to those gas stations identified in 

the TPSS study as having working air 

pumps equipped with pressure gauges. 

The twin purposes of the APGAS were: 

1) to evaluate the accuracy –at specified 

pressure levels (25, 35, 45, 55 psi)-- of 

the station’s pressure gauge; and, 2) to 

identify the type of gauge used. Thus, 
National Center for Statistics and Analysis 400 Seventh St. S.W., Washington, DC 20590

in this report the data regarding the 

measurement of gauge accuracy come 

from the APGAS, while all of the other 

data come from the TPSS.

Survey Methodology

In the TPSS, the gas stations were 

sampled using a multi-stage selection 

process1 whose goal was to obtain a 

sample of gas stations -from all regions 

of the country- that were as 

representative2 as possible.

The APGAS was designed to ensure 

that the measurement and gauge 

identification data were as accurate as 

possible by controlling for:

i. the equipment used;

ii. time of day at which 

measurements were taken;

iii. pressure levels at which the 

reference pressures were set; and

iv. the procedure followed to take the 

pressure measurements.

At each gas station, an identical device 

(Air Works 7 gallon Portable Air Tank) 

was pressurized using the station’s 

pump to reference pressure levels 

(measured using the station’s gauge ) of 

25, 35, 45 and 55 psi. For each of these 

levels, a corresponding measurement 

was recorded using the researcher’s 

gauge (in every case, the same model 

 

1

 For details, see Research Note, Tire Pressure 

Special Study: Methodology, NHTSA, August 2001, 

DOT-HS-809-315.

2

 However, please note that, since the population 

from which these gas stations were drawn consisted 

of those having at least two islands, all of the 

estimates reported here refer to gas stations of at least 

this size. 

of gauge was used: a Longacre Model 

50402 0-60 psi Tire Pressure Gauge).

The uniform procedure consisted of:

i. taking the measurements in the 

above order;

ii. for each level, first bleeding the 

tank to at least 20 psi less than the 

reference level3; 

iii. refilling the tank to the reference 

level (as measured by the station’s 

gauge); and, then,

iv. measuring the actual pressure 

using the researcher’s gauge.

iv. The day’s second set of 

measurements was then obtained by 

repeating this same process in the 

same order. 

In addition to ensuring uniformity of 

procedure and equipment, a special 

effort was made to make recorded 

measurements as independent as 

possible by: 

i. taking 4 different readings at 

each reference pressure level; 

ii. having the readings taken on two 

different days (two each day) by

iii. two different researchers who 

were instructed to

iv. fax his/her readings to a NASS 

Zone Center immediately after taking 

them and to

v. not show or discuss his/her 

measurements with the other 

researcher. 

 

3

 For example, to obtain the 45 psi measurement, the 

tank would first be bled down to below 25 psi.
National Center for Statistics and Analysis 400 Seventh St. S.W., Washington, DC 20590

Finally, the type of gauge the station 

had was identified by taking a digital 

photograph of the gauge, which was 

then emailed to NHTSA where it’s type 

could be unambiguously determined.

Results: Pump Availablility

The TPSS results show that it is quite 

common for gas stations to have air 

pumps: from these data NCSA 

estimates that 94 percent of the 

stations in the US have air pumps. 

Although this figure indicates that, in 

general, when a passenger vehicle pulls 

into a gas station the driver can feel 

fairly confident of encountering an air 

pump, it could be masking a possible 

problem in the sense that stations which 

have small amounts of traffic 

- specifically those in sparsely populated 

areas- may have a much lower 

probability of being equipped with an air 

pump.4 Thus, it would be desirable to 

get some idea of to what extent pump 

availability varies with station traffic. As 

Chart 1 shows, there appears to be 

strong positive relationship between 

pump availability and station traffic. 

However, even at the lowest traffic level5

 

4

 Since urban areas tend to be well-endowed with gas 

stations, even if only 80 percent of the stations have 

air pumps, the consumer should have little difficulty 

in finding an air pump equipped station. On the other 

hand, a similar percentage in rural areas could point 

to exactly the opposite conclusion: since there are 

many fewer gas stations available, if only 80 percent 

of them have pumps, this may mean that some 

consumers will have a difficult problem finding air 

pumps. 

5

 The TPSS survey provided the estimates for station 

traffic by counting the number of vehicles coming in 

to the gas pumps during five 15 minute periods 

starting at 8 am, 10 am, 12 pm, 2 pm and 4 pm. Thus, 

the station traffic was arrived at by taking the sum of 

the traffic during these five 15 minute periods. For 

this report, the data was used to partition the stations 

into 7 traffic levels as shown in Table 1.

(accounting for only 14 percent of the 

gas stations in the US), the number of 

stations equipped with pumps is 

estimated to be over 85 percent. 

Results: Pump Functionality

Pulling into a gas station and finding an 

air pump there is one thing, but what if 

it’s not working? Are non-working 

pumps a problem, or can the consumer 

feel reasonably sure that if the station 

has a pump that it will be functioning? 

The data show that an estimated 9 

percent of gas station air pumps will not 

be working. Thus, when the consumer 

pulls into a gas station the probability 

that a pump will be there and also that it 

will be functioning is about 86 percent.

 

Table 1

Criteria Used to Partition Stations

into Traffic Categories

Traffic 

Level

Number of Vehicles 

in 75 mins

% of Stations in 

Traffic Level

1 0 –15 14%

2 16 - 25 18%

3 26 - 35 23%

4 36 -45 20%

5 46 -60 11%

6 61 - 75 10%

7 76 + 4%
National Center for Statistics and Analysis 400 Seventh St. S.W., Washington, DC 20590

The next step is to condsider what 

factors play a role in determining 

whether a pump is working or not. 

Since many stations charge a fee for 

use of the air pump, it is reasonable to 

investigate the relationship between 

whether a fee is charged and the 

probability that that pump will actually 

be working. A priori, one would think 

that there should be a positive 

relationship between the presence of a 

pump fee and the probability 

- conditional on a pump being availablethat it will actually be working: the 

reason being that if a fee is charged, 

then the owner of the pump has more of 

an incentive to keep it working. 

Although such a hypothesis seems 

reasonable, the actual results show the 

opposite: the presence of a fee actually 

lowers the probability that the pump will 

be working: from 95 percent if no fee is 

charged, to 86 percent in the presence 

of a fee.

Results: Gauge Availablility

Maintaining proper tire pressure 

requires not just using a pump, but also

measuring tire pressure. Thus, it is 

significant that less than half (49 

percent) of the pump-equipped gas 

stations also have a tire gauge. 

The next thing to consider is whether 

this has something to do with the fact 

that most stations (57 percent) do not 

charge a fee for pump use. And, 

indeed, there is a strong relationship 

between gauge availability and pump 

fees. As Chart 2 illustrates, in the 

presence of pump fees, estimated 

gauge availability (conditional on a 

pump being available) more than 

doubles, going from only 31 percent to 

68 percent. 

Results: Gauge Accuracy

One of the most critical items this study 

set out to discover was exactly how 

much faith the consumer could place 

on the pressure readings given by the 

station’s gauge. 

Since the rulemaking goal specified by 

the TREAD Act concerned coming up 

with a system to report under-inflation, 

clearly Congress felt that under-inflation 

was a more dangerous problem than 

over-inflation. The APGAS found that, 

in general, gas station gauges tend to 

over-report, rather than under-report, 

the true tire pressure (thus encouraging 

the more dangerous under- rather than 

the less dangerous over-inflation). 

Table 2 and Chart 3, which show the 

distribution of the deviations between 

the station gauge and the reference 

gauge for the four reference levels, 

illustrate this tendency to over-report.
National Center for Statistics and Analysis 400 Seventh St. S.W., Washington, DC 20590

Table 2

Amount of Deviation (psi) of Station Gauge from Reference Gauge

By Percentiles*

Percentiles Station 

Gauge 

Pressure

(psi)

5th 10th 20th 25th 40th 50th 60th 75th 80th 90th 95th

25 -7.3 -4.5 -1.3 -0.7 0 0.3 0.7 2 3.1 4.6 5.3

35 -5 -3.5 -0.4 0 0.5 0.8 1.4 2.5 3 5.4 6

45 -5.9 -3.3 0 0 0.8 1 1.7 3 3.1 6.3 8

55 -2.7 -1 -0.2 0 1 1.5 2.3 3.4 3.9 6.8 8.1

*

Note: Negative values indicate under-reporting: measurement of the station gauge is less than the researcher’s gauge. 

Positive values indicate over-reporting: measurement of the station gauge is greater than the researcher’s gauge.

Source: National Center for Statistics and Analysis, NHTSA, APGAS Survey, Aug. 2001

The APGAS also found that the higher 

the recommended tire pressure, the 

greater the probability of encountering a 

station whose gauge will over-report the 

actual pressure (leading motorists to 

under-inflate their tires). For example, at 

a pressure level of 25 psi, 25 percent of 

station gauges over-report pressure by at 

least 2 psi; at pressure levels of 35 and 

45 psi, about 32 percent of station 

gauges over-report by this amount; while 

at 55 psi, about 45 percent of station

gauges over-report pressure by at least 2 

psi.

While over-reporting of 2 psi may not be 

significant, many station gauges did over 

report by a substantial amount. Table 3 

shows the percentage of station gauges 

over-reporting by 4 psi or more.

Table 3

Over-Reporting by Station 

Gauges

By Pressure Levels and Level of 

Over-reporting 

(Percent Over-reporting) 

Station Percent over-reporting

Gauge 

Pressure

(psi)

by 4 

psi or 

more

by 6 

psi or 

more

by 8 

psi or 

more

25 16 2 0

35 19 9 0

45 19 14 5

55 20 15 9

Source: National Center for Statistics and Analysis, 

NHTSA, APGAS Survey, Aug. 2001

As Table 3 shows, at a pressure level of 

45 psi, 5 percent of station gauges over-
National Center for Statistics and Analysis 400 Seventh St. S.W., Washington, DC 20590

report tire pressure by 8 psi or more; 

while at a pressure level of 55 psi, overreporting of this magnitude occurs at 

nearly one station in ten. 

Between (depending on the pressure 

level) 16 and 20 per cent of gas station 

gauges over-report pressures by at least 

4 psi (Table 3). Thus, for about one gas 

station in five, it will be the case that a 

motorist filling his tires to the 

recommended pressure will, in fact, have 

unknowingly under-inflated them by at 

least 4 psi. 

At 35 psi, which is at or close to the 

recommended tire pressure for most 

passenger cars and SUVs, 9 percent of 

station gauges over-report by 6 psi or 

more. That is, at nearly one station in 

ten, the motorist relying on the station 

gauge to fill the tires of his car or SUV to 

their recommended pressure will, in fact, 

be unknowingly under-inflating them by 6 

psi or more.

Given these findings, the question arises 

as to whether a motorist would be better 

off bringing his own gauge. NHTSA’s 

Vehicle Research and Test Center 

(VRTC) tested 17 new6 commercial 

gauges and found that, as was the case 

with gas station gauges, hand held 

gauges tend to over- rather than underreport pressures (thus encouraging 

under-, rather than over-, inflation). Mean 

deviation from the reference gauge 

ranged from 0.6 psi (at a pressure level of 

55 psi) to 1.6 psi (at a pressure level of 

25 psi). Five percent (Chart 4) of the 

hand held gauges over-reported 

pressures by at least 4 psi (pressure 

level, 25 psi) while, at the other pressure 

levels, between 2.5 percent and 0 

 

6

 Thus, unlike the gas station data, the hand gauge data 

does not reflect the impact of wear and tear on gauge 

accuracy.

percent of the hand held gauges overreported by at least 4 psi.

Finally, the 17 gauges were partitioned7

into digital and non-digital (i.e., dial and 

pencil) gauges. The mean deviations for 

non-digital gauges ranged from 1.3 psi 

(pressure levels, 35-45 psi) to 1.8 psi 

(pressure level, 55 psi); in contrast, digital 

gauges showed a mean deviation of 0.1 

to 0 psi at all pressure levels.

Results: Pump Fees

Overall, an estimated 43 percent of the 

pump-equipped gas stations in the US 

charge a fee for use of their air pump. 

However, this study found that whether or 

not a fee is charged varies greatly (Chart 

5) from region to region. 

 

7

 The measurement mechanism of dial/pencil gauges is 

fundamentally different from that of digital gauges: 

the latter is mechanical while the former is electronic 

(Chip Chidester/Art Carter, NHTSA).
National Center for Statistics and Analysis 400 Seventh St. S.W., Washington, DC 20590

In the West only 18 percent of the gas 

stations charge a fee for pump use, while 

in the South, fees are charged 63 percent 

of the time. 

Conclusions 

1) Overall, the probability that a randomly 

selected gas station will have an air pump 

is 94 percent. 

2) Pump availability does vary positively 

with the volume of traffic through a gas 

station: the higher the traffic, the more 

likely it is to have an air pump (there 

appears to be a high floor for this 

percentage: even among those stations 

in the lowest quintile of volume, the 

probability that they would have a pump 

available is still in excess of 85 percent). 

3) Overall, the probability that a randomly 

selected gas station will have a 

functioning air pump is 86 percent. 

4) Surprisingly, if a fee is charged for use 

of a pump, the probability is greater that 

the pump will not be working.

5) Given that a station has an air pump, 

the estimated probability that that air 

pump will also have a gauge8 is only 

around 50 percent. 

6) The presence of a pump fee 

substantially increases the probability that 

a station will have an air pump equipped 

with a tire pressure gauge.

7) Station gauges tend to over-, rather 

than under-, report the true tire pressure 

(thus encouraging the more dangerous 

under- rather than the less dangerous 

over-inflation). 

8) The magnitude by which station 

gauges over-report tire pressure is 

significant: between 16 percent and 20 

percent of station gauges over-report by 

at least 4 psi (all pressure levels); for 

pressures of 35-55 psi, between 9 

percent and 15 percent over-report by at 

least 6 psi while for pressure levels of 45-

55 psi, between 5 percent and 9 percent 

of station gauges will over-report by 8 psi 

or more. 

9) Like station gauges, hand held gauges 

tend to over- rather than under-report tire 

pressures (thus leading motorists to 

under-, rather than over-, inflate their 

tires). Nevertheless, hand held gauges 

were not only much less likely to overreport tire pressures by 4 psi or more but, 

unlike station gauges, none of the hand 

helds over-reported by more than 8 psi.

10) Overall, about 43 percent of U.S. gas 

stations charge a fee for pump use. 

11) The probability that a fee will be 

charged for pump use is regionally 

dependent (varying from a low of 18 

percent in the West to a high of 63 

percent in the South). 

 

8

 I.e., the probability that a gas station will have a tire 

gauge conditional on it having an air pump.
Concluding Note

With respect to tire gauges, two things 

must be emphasized. First, only about 50 

percent of station pumps are equipped 

with a gauge. Secondly, gas station 

gauges tend to over-report tire pressures 

(i.e., give readings which exceed the 

actual pressure). Consequently, 

motorists who rely solely on a station 

gauge to evaluate their pressure face a 

high probability of unknowingly underinflating their tires. For both these 

reasons it is advisable that motorists 

bring and use their own gauge. 

How seriously could they be underinflating their tires? It depends on the 

pressure to which they inflate their tires. 

In the 25-35 psi range (the recommended tire pressure range for virtually all 

passenger cars and SUVs) nearly one 

station gauge in five will over-report tire 

pressure by at least 4 psi. At 35 psi, 

nearly one station in ten will give a 

reading which exceeds the actual 

pressure by at least 6 psi (thus, a motorist 

inflating his tires to a recommended 

pressure of 35 psi will, in fact, pull out of 

such stations with an actual pressure of 

29 psi or less).

Finally, most drivers do not randomly 

select the gas station at which they check 

their tire pressures -instead, they 

probably have a particular gas station 

which they frequent and at which they 

check their tire pressure. Thus, without 

even realizing it, at about one station in 

five it is the case that drivers who rely on 

the station’s gauge to check their tire 

pressure are using a gauge that overreports the pressure by 4 psi or more, 

thus causing them to under-inflate their 

tires by this amount.

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 Joseph Stevano [202-366-2219] 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-31

Washington, D.C. 20590