NHTSA Effects update

This report presents an analysis of the data collected through the Tire Pressure Monitoring System-
Special Study (TPMS-SS) as it pertains to the effectiveness of TPMS in promoting proper tire inflation.
The TPMS-SS was conducted in 2011, using a nationally representative sampling structure based on the
primary sampling units (PSUs) of the National Automotive Sampling System. NASS personnel collected
6,103 complete vehicle observations (4,391 of which were equipped with TPMS) including tire pressure
and temperature of all four tires. Only vehicles in the model year range 2004-2011 were surveyed.
Proper tire inflation is important for several reasons. Underinflated tires experience a greater amount
of sidewall flexion than properly inflated tires, resulting in decreased fuel economy, sluggish handling,
longer stopping distances, increased stress to tire components, and heat buildup that can lead to
catastrophic failure of the tire, such as cracking, component separation, or blowout. These catastrophic
failures can cause loss of vehicle control and may result in a crash. Overinflated tires may be more easily
damaged by potholes or debris. Severe overinflation may increase stopping distance due to reduced
area of road contact and non-optimal traction, and may also contribute to vehicle instability. As with
underinflation, overinflation may result in uneven tread wear that reduces the useful life of the tire.
This report does not attempt to directly measure the relationship between TPMS and vehicle safety;
rather it measures the relationship between TPMS and proper tire inflation. In order to estimate the
effect that TPMS has on crash avoidance and mitigation, future analyses are planned that will use realworld
crash data.
This survey found that 12.4 percent of all passenger vehicles in the US of model years 2004-2011 have at
least one tire that is severely underinflated as defined by FMVSS No. 138 (25% or more below the
vehicle manufacturer’s recommended cold tire pressure). The survey also found that 23.1 percent of
the MY 2004-2007 vehicles without TPMS had at least one severely underinflated tire, but only 11.8
percent of the MY 2004-2007 vehicles equipped with TPMS – and only 5.7 percent of the more recent,
MY 2008-2011 vehicles equipped with TPMS – had a severely underinflated tire. Based on the data
from model year 2004-2007 vehicles (the range of collected model years that contained both vehicles
with and without TPMS), TPMS was estimated to result in a statistically significant 55.6-percent
reduction in the likelihood that a vehicle will have one or more severely underinflated tires.
TPMS is also estimated to result in a 30.7-percent reduction in the likelihood of severe overinflation
(25% or more above the manufacturer’s recommended cold tire pressure) for model year 2004-2007
vehicles. This effect was present in vehicles with TPMS systems that do not alert the driver to
overinflation, and it is unclear what causes the association between TPMS and reduced overinflation.
Although the specific effects of overinflation are not well documented, it is reasonable to assume that it
results in more rapid tire wear and, possibly, reduced vehicle stability.
While the primary goal of TPMS is to reduce underinflation in order to make vehicles safer to operate, a
further benefit of reduced underinflation is improved fuel economy. By combining estimates of reduced
underinflation due to TPMS with estimates of increases in fuel economy resulting from increases in tire
pressure, it’s possible to estimate the amount of fuel that TPMS will save an average vehicle during a
given period of time. During the first eight years of operation, TPMS is estimated to save a typical
passenger car 9.32 gallons of fuel and a typical LTV 27.89 gallons of fuel. During 2011 TPMS is estimated
to have saved $511 million across the vehicle fleet through reduced fuel consumption. This estimate
does not include any additional savings that may result from extended tire life or any crash‐avoidance
benefits.
Future analyses of TPMS effectiveness will be conducted using crash data to directly measure the
relationship between TPMS and tire‐related crashes. Also, future analyses are planned to analyze the
interview data collected through the TPMS‐SS. This interview data includes information about driver
behavior as it pertains to tire care and maintenance.