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A statistical study of tire pressures on road going vehicles.


Published information on studies of something so critical to safety as passenger vehicle tire pressures can be found [1, 2]; however, they only account for rolling tires. Studies related to spare tire pressures are lacking. This paper is the result of measurements on 150+ vehicles and the most surprising results are presented regarding the influence of Tire Pressure Monitoring Systems (TPMS) and the new spare tire locations and use. A statistical study was performed on the collected data to determine the correlation between tire pressures, vehicle age and TPMS. One particular topic of investigation was the relationship between various factors that influence spare tire pressure. Some newer models, particularly some mini-vans, have placed the spare tire in an unusual and inconvenient place for regular maintenance. Based on the data collected, TPMS has a positive influence on rolling tires but not on spare tires. The results support the need for TPMS to also monitor spare tire pressures.

CITATION: Popat, J., Nabar, A., Read, M., Fu, C. et al., "A Statistical Study of Tire Pressures on Road Going Vehicles," SAE Int. J. Passeng. Cars - Mech. Syst. 9(2):2016.


For something as critical to passenger vehicle safety as spare tire pressures, public information is seriously lacking. This paper is the result of measurements on more than a hundred vehicles and the most surprising results are presented regarding the influence of TPMS and the new trend toward spare tire locations and use.

Gathering this data is notoriously difficult due to the psychological hurdles involved in gaining access to a trunk that can be embarrassingly cluttered and the time required to measure a tire buried under floor boards. One key observation is that spare tires are often left unchecked or overlooked in between regular vehicle maintenance resulting in safety issues for drivers.

The vehicles sampled ranged from full-sized pick-up trucks to compact cars. The study included vehicle model years from 1965 to 2015 (50 year range). The sampling was performed at the University of North Carolina at Charlotte campus as well as at local tire shops. The tire pressures were measured, recorded, and typically corrected to the manufacturer recommended pressures.

A statistical study was performed on the collected data to determine the correlation between tire pressures versus vehicle age, etc. One particular topic of investigation was the relationship between spare tire location and spare tire pressure. Some newer models, particularly some mini-vans, have placed the spare tire in an unusual and inconvenient place for regular maintenance. Based on the data collected, older vehicles were more likely to have lower spare tire pressures than their newer counterparts, regardless of the ease of access to the spare tire and TPMS has a positive influence on rolling tires but not necessarily spare tires.


On higher end vehicles, there is a trend towards more "Run-Flat" tires. These tires cost more, the vehicle ride is harsher and requires more compensation, and they will not be useful if there is rim damage. With stiffer sidewalls, run flat tires need a TPMS system to determine if there is air loss. Also, they have limited range once they are flat. They can only be safely driven for fifty miles at speeds less than fifty miles an hour [3, 4]. Even then, they can be damaged and need replacement. For these reasons, spare tires are still the norm.

While some vehicles equipped with a spare tire carry full-size spare tires, most now carry temporary spare tires. Temporary spares are smaller in size and construction from the main tires and wheels on the vehicle and typically request 60 pounds per square inch (psi) inflation pressure and maintain the full size tire diameter. However, they are smaller in volume than the main tires to lower the vehicle's weight, and save space for a larger storage compartment. As a result of their rare need and to reduce costs, they are often made to a lower quality standard.

Because the spare tire serves as an essential part to the reliability and safety of a road vehicle, regular maintenance is crucial to efficient performance of the spare tire. However, temporary spare tires have lower internal volume so a small air loss results in a greater lowering of the pressure. In addition, air loss is a function of pressure and since the temporary spare almost always requires 60 psi, it loses air faster than a tire with less pressure. Typical tires lose air about 1 psi per month [5] whereas we have measured temporary spare tire lose at over twice this rate. This higher pressure and lower volume make the temporary spare more vulnerable to under-inflation.


In the Department of Transportation (DOT), the National Highway Traffic Safety Administration (NHTSA) implemented the Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act in April of 2005. In this, a mandate was made for vehicles to have a Tire Pressure Monitoring System (TPMS) which warned the driver of a low tire pressure.

Complaints about TPMS on spare tires included:

1. Spare tire TPMS would cause low pressure faults with spare tire use.

2. Requiring spare tire TPMS would motivate manufacturers to eliminate spare tires.

3. Spare tires are difficult to monitor on wheel speed systems.

4. Battery powered spare tire TPMS systems would require three million additional batteries in the environment.

Unfortunately, after much discussion regarding the impact of spare tire TPMS on the discontinuity of spare tires, the TREAD Act does not require monitoring of the spare tire.


Despite the TREAD Act requirement that a statement is added to owner's manuals that "Each tire, including the spare (if provided), should be checked monthly" [6], there is a risk for spare tire under-inflation.

Vehicle manufacturers are finding that the spare tire is in a very useful location for storage. To maximize this space, they have made several changes that make it difficult to check the spare tire pressure.

* Temporary spares stored in the trunk are installed valve stem down to free up wheel cavity space.

* On pickup trucks and SUV's, spare tires are stored up under the back of vehicles with the valve stem up.

* Temporary spares on many mini-vans have started moving them to up under the vehicle between the front seats and with the valve stem up.

The last two require assembling a hand crank and then winching the tire down to gain access.

On a positive note, one sample vehicle, a MINI Cooper, the spare was attached to a hose which provided a pressure tap for a TPMS system and a 'T' out to a remote valve stem near the rear bumper. It was easy to check and inflate, allowed the TPMS to keep tabs on the spare tire, and disconnected easily from the spare for removal.

The spare tire should be checked for punctures and cracks and inflated regularly. However, spare tires are often overlooked or left unchecked between regular vehicle maintenance resulting in a safety issue for drivers. This omission can be attributed to difficulty of access, such as having to empty the contents of one's trunk, and reliance on road-side assistance in the event of a damaged or flat tire.


Three general locations were used for gathering tire pressure data. The first was a "free air" pump on the campus of the University of North Carolina at Charlotte. In order to measure tire pressures we found that considerable psychological effort was needed to convince people to allow us the opportunity. About half the spare tires were in trunks that were half filled with items that would be best described as 'clutter' and these drivers did not want us to even see into the messy trunk. Many older cars either contained no spare or had no way of allowing the tire removal. Several vehicles that require assembling a crank to access the spare did not have the crank mechanism. On our first experience with the central storage location, the driver of a newer mini-van wouldn't allow us near the spare tire as he knew the time penalty involved.

The second location was the Firestone tire store on North Tryon St. in Charlotte, NC where the generous manager allowed us access as a research project (Thank you). We found the same rate of "junk in the trunk" and missing tools. However, as a rule, if you bring a car in for tire work, they have legal access to your trunk. Half of the tire pressure measurements were made this way and we worked hard to not get in the way of any tire technicians.

The third location would best be described as at "friends and family". About a dozen mechanical engineering students took calibrated pressure gages with them and measured pressures at their apartments and while on break. These students were recruited from a senior level "Tire Mechanics" course as extra credit. The students worked in pairs, used calibrated gages, and as needed, used little air pumps and replaced missing valve caps (black, gray and green).


Check sheets were used to record vehicle information. We recorded the name of the person measuring; the date, time, and temperature. We also recorded the vehicle type, make, model, and year. On the tires, we recorded the brand and size recommendations. If the values were in a safe range, the tire pressures were set to the driver preferred values. The safe range lies between the vehicle manufacturers suggested set point and maximum inflation pressure as noted on the sidewall of the tire. The pressures were always set closer to vehicle manufactures recommended pressure.


Most of the pressure data presented in this paper was collected between 10 am and 4 pm on mostly sunny days, with an average temperature between 21 and 32[degrees]C (70 and 90[degrees]F) to keep the effect of temperature variance in check. Tires will rise about 0.1 bar (1.5 psi) for every 10[degrees]C (18[degrees]F) of air temperature change [1, 7]. In accordance with the findings of [1], tire pressures were set to a higher value depending on the running and time of day.


Bourdon tube air pressure gauges were used to measure tire pressures at both locations. The gauges were calibrated prior to testing to ensure accurate and consistent data collection using an AMETEK TQ-100 Deadweight Tester [3] shown in Figure 1. All tire pressures, including the spare tire, were measured and recorded, along with the vehicle manufacturer, model, year, and the designation and DOT code of the tires. After measurement, the under- or over-inflated tires were set to the (corrected) manufacturer recommended values found on the driver-side doorjamb of the vehicle. A small portion of vehicle owners requested tire pressure values different from the manufacturer. After ensuring that these owner requested pressures were in the safe range and closer to vehicle manufacturer recommended pressure, the pressures were set.


The data obtained through testing yielded 150+ complete vehicle samples. The current and manufacturer recommended spare tire pressures were compared for all samples, and the percent under inflation of the spare tire was calculated using these values. As mentioned before, data was obtained from the "free air" pump, firestone tire shop and friends and family of the students involved. A characteristic that most of the vehicles shared was that they had not been driving long distances before the measurements were taken. This negated the need to apply temperature corrections to the data. No other processing is done and the data is used as is. While the model years of the vehicles tested ranged 50 years, the majority of the vehicles tested were manufactured after 1990. Figure 2 presents a scatterplot of the data obtained for each sample model year and the corresponding percent under inflation of the spare tire. Out of all vehicles tested, only five had properly inflated spare tires based on manufacturer recommendations. The remaining spare tires were underinflated whose analysis follows.

Of the vehicles sampled, 114 had spare tires that were more than 10 psi below manufacturer recommendations. This equates to 73.5% of the total vehicles tested having significantly insufficient spare tire pressure. Figure 3 provides the spare tire percent under inflation distribution for all vehicles sampled. The average percent under inflation of the spare tire for all vehicles was 47%. This value is alarming as such low pressure will render the spare tire useless in the event of an emergency situation because it might not be safe to carry the load.

In June 2014, IHS Automotive released their yearly study regarding the average age of light vehicles on the road in the United States. The study, which sampled vehicles in January of 2014, found the average age of all light vehicles on the road was 11.4 years old. This number remained steady from 2013, and is expected to rise slightly to 11.5 by the year 2017 [8]. Based on this study, the average vehicle on the road in 2015 was manufactured in 2004. To see if vehicle age affected the spare tire pressure, the data was divided in two parts according to vehicle model year using the average vehicle model year of 2004 as the dividing line. The first division included the model years of 2004 and earlier, while the second division included model years later than 2004 up to the current year of 2015. The samples were nearly evenly split, with the first and second divisions having 71 and 84 vehicle samples, respectively.

For vehicle model years of 2004 and earlier, average percent under inflation of the spare tire was 51.9% as shown in Figure 4. Vehicles with model years after 2004 fared better than their older counterparts. The average percent under inflation for the newer vehicles was 42.8% as shown in Figure 5.

Although the average percent under inflation for the older vehicles was approximately 9% higher than newer vehicles, the overall probability of the older vehicles having an underinflated spare tire was only 2% higher than newer vehicles. This shows that even the spare tires of newer vehicles are often neglected during maintenance.

A comparison between the newer and older model years at various percent under inflation rates of the spare tires was made using the test data. The three spare tire under inflation rates investigated include 10% and over, 25% and over, and 50% and over. The results for each model year division are provided in Table 1.

In each model year division, over 80% of the vehicles tested had a spare tire more than 10% underinflated based on the manufacturer recommendations. As the under inflation rate increased to 25% and above, the probability of under inflation for each division decreased to approximately 70% for both divisions. At 50% under inflation and above, the probability of under inflation for each division decreased; however, a very noticeable gap between the older and newer vehicles became evident. Older vehicles with model years of 2004 and earlier are 1.25 times more likely to have a spare tire underinflated by 50% or more compared to newer vehicles. This is significant as older model years make up a large portion of the vehicles on the road. With 52% of these older vehicles having spare tires that are 50% or more underinflated, drivers face a considerable safety risk in the event of a tire failure.

Analysis was also done to understand the impact of TPMS on tire pressures for both rolling and spare tires. Since, most of the vehicles tested were front wheel drive, we use the left front tire as a reference for comparison with the spare tire. In other words, a driven tire was chosen over a non-driven tire because it deals with greater load. It was also decided to use vehicles that were older than two years to account for pressure loss with time. New vehicles that undergo maintainance under warranty might also have spare tires which were checked and set to the recommended pressure. This exclusion of newer vehicles deleted the possible bias in the data which primarily consists of vehicles whose spare tire pressures weren't checked over a period of time.

The analysis was done using 126 complete vehicle samples out of which 54 vehicles were equipped with TPMS and 72 vehicles didn't have TPMS. In Figures 4 and 5, error in pressure is the difference between manufacturer recommended pressure and measured pressure. The average value of error in pressure for the LF tire in vehicles without TPMS is 5psi as compared to those with TPMS at 3.8psi. The authors believe that additional vehicles measured at different times of the year would lead to a greater difference in these numbers. However, a difference of almost 5% (additional 2.1psi for a tire rated at 35psi) might be enough to tip the tire over its nominal working range of pressure. This in turn can hurt fuel economy and ride quality. Figure 5 depicts the error in tire pressure and impact of TPMS for the spare tire. It is found that the average difference between recommended and recorded spare tire pressure values are almost equal at 27 psi, irrespective of whether the vehicle is equipped with TPMS or not. In terms of percent under-inflation, it is close to 48% which renders the tire useless in emergency situations as stated earlier. Thus, the positive impact of TPMS is clearly visible on rolling tires. It reaffirms the effectiveness of TPMS established in [2],albeit with a small sample space. However, the high percent under-inflation of spare tires further supports the need for TPMS to be connected to the spare tire.


A study of driving and spare tire pressures from various road vehicles was completed in order to investigate the relationship between spare tire pressures and vehicle age. Out of 155 vehicle samples, 73.5% had spare tires that were at least 10 psi below manufacturer recommendations. Since the average vehicle on the road in the United States was manufactured in 2004, the data was separated into two divisions to investigate the spare tire pressure trends of older and newer vehicles with the model year of 2004 as the dividing line. It was discovered that vehicles with model years of 2004 and earlier are 1.25 times more likely to have a spare tire 50% or more underinflated compared to newer vehicles. In addition, the average under inflation rate for the older vehicles was 9% higher than newer vehicles. Although older vehicles proved to be more likely to carry spare tires that were significantly underinflated compared to the newer models, 90% of all vehicles tested were uninflated by some amount compared to manufacturer recommendations. This presented an argument for the TPMS to also monitor spare tire pressures. Further analysis on the basis of presence of TPMS revealed a percent under-inflation of 16% of vehicles without TPMS and 11.7% for vehicles with TPMS for the left front tire. For the spare tire, this number increases to 48% which further emphasized the need for TPMS systems to be connected to the spare tire.

The analysis showed that both newer and older models carry spare tires that need servicing due to air pressures below manufacturer recommendations. In order to alleviate safety concerns for drivers, spare tires should be checked during every regular vehicle maintenance appointment and when the four main tires are checked for proper inflation. The NHTSA recommends checking pressures of all tires, including the spare tire, once every month [9]. Although many drivers rely on road-side assistance or inflation and sealant kits, these options may not always be available or the best choice given an emergency situation. For driver safety, spare tire maintenance is essential.

Based on the data collected for this paper, low spare tire pressures were independent of spare tire location. However, this fact should be revisited in the future. Currently, there are vehicles on the road with very difficult access to the spare tire. In particular, newer model minivans place the spare tire beneath the center console and require a wrench be used inside the vehicle to lower the spare. This is inconvenient, as drivers need to clear out the inside of the vehicle and be careful not to damage the radio while turning the wrench in order to access and lower the spare. Since these are newer models, the spare tires have not lost much air from the factory. However, it would be interesting to investigate if, in a few years, the access difficulty will influence the spare tire pressure and present a problem for the drivers of these vehicles.


[1.] NHTSA's Tire Pressure Special Study. Thiriez, Kristin and Bondy, Nancy. 2001, In Proc., 18th International Technical Conference on the Enhanced Safety of Vehicles.

[2.] Sivinski, Robert. Evaluation of the Effectiveness of TPMS in Proper Tire Pressure Maintenance. s.l. : DOT, 2012. No. DOT HS 811 681. 2012..

[3.] Model Type T Hydraulic Deadweight Tester Specification Sheet SS-2150-T. Rockchester, NY : Ametek, 2007.

[4.] Driving on Flat Run-Flat Tires. Tire Tech. [Online] Tire Rack. [Cited: 11 13, 2015.]

[5.] Mercedes-Benz Operator's Manual E-Class Sedan. Germany : Mercedes-Benz USA, LLC, 2005.

[6.] TRANSPORTATION, DEPARTMENT OF. Federal Motor Vehicle Safety Standards; Tire Pressure Monitoring Systems. National Highway Traffic Safety Administration 49 CFR Parts 571 and 585. [Online] April 8, 2005.

[7.] Popat, Jugal. "Baking the Tire" Research Project. Charlotte, NC : UNC Charlotte, 2014.

[8.] "Stepney Spare wheel, made for early years.". [Online] April 29, 2015.

[9.] Tire Maintenance. Tirewise. [Online] National Highway Transportation Safety Administration. [Cited: 11 13, 2105.]

[10.] "Average Age of Vehicles on the Road Remains Steady at 11.4 years, According to IHS Automotive.". [Online] April 29, 2015.


The corresponding author is Peter Tkacik at


The work of this paper was made possible by the Motorsports and Automotive Research Center (NCMARC) at the University of North Carolina at Charlotte.


TPMS - Tire Pressure Monitoring System

psi - Pounds per square inch

DOT - Department of Transportation

NHTSA - National Highway Traffic Safety Administration

TREAD - Transportation Recall Enhancement, Accountability, and Documentation

Jugal Popat, Aneesh Nabar, Meighan Read, Chen Fu, Chunhui Zhang, Galab Kausik, Harsh Patel, and Peter Thomas Tkacik

University of North Carolina

Table 1. Probability of spare tire under inflation for both vehicle
model year divisions

% underinfiated  Vehicle Model Year
                 2004 and Older  Newer than 2004

>10%             85.9%           84.5%
>25%             69%             71.4%
>50%             52.1%           41.6%
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Author:Popat, Jugal; Nabar, Aneesh; Read, Meighan; Fu, Chen; Zhang, Chunhui; Kausik, Galab; Patel, Harsh; T
Publication:SAE International Journal of Passenger Cars - Mechanical Systems
Article Type:Report
Date:Jun 1, 2016
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