The agency has received data from Goodyear indicating that significantly under-inflated tires increase a vehicle's stopping distance. ⁽²¹⁾ The effects of tire under-inflation on vehicle stopping distance are discussed in greater detail in the agency's Final Economic Analysis (FEA).

As explained in the FEA, the agency did not use the VRTC data or the Goodyear data that the agency used to estimate benefits in the NPRM because of concerns with the way in which the both tests were performed. ⁽²²⁾ The agency believes that the more recent Goodyear test methodology adequately addressed these concerns. ⁽²³⁾

2. Reduced Tread Life

Unpublished data submitted to the agency by Goodyear indicate that when a tire is under-inflated, more pressure is placed on the shoulders of the tire, causing the tread to wear incorrectly. ⁽²⁴⁾ The Goodyear data also indicate that the tread on an under-inflated tire wears more rapidly than it would if the tire were inflated to the proper pressure.

The Goodyear data indicate that the average tread life of a tire is 45,000 miles, and the average cost of a tire is $61 (in 2000 dollars). Goodyear also estimated that a tire's average tread life would drop to 68 percent of the expected tread life if tire pressure dropped from 35 psi to 17 psi and remained there. Goodyear assumed that this relationship was linear. Thus, for every 1-psi drop in tire pressure, tread life would decrease by 1.78 percent (32 percent / 18 psi). This loss of tread life would take place over the lifetime of the tire. Thus, according to Goodyear's data, if the tire remained under-inflated by 1 psi over its lifetime, its tread life would decrease by about 800 miles (1.78 percent of 45,000 miles).

As noted above, data from the NCSA tire pressure survey indicate that 26 percent of passenger cars had at least one tire that was under-inflated by at least 25 percent. The average level of under-inflation of the four tires on passenger cars with at least one tire under-inflated by at least 25 percent was 6.8 psi. Thus, on average, these passenger cars could lose about 5,440 miles (6.8 psi under-inflation x 800 miles) of tread life due to under-inflation, if their tires were under-inflated to that extent throughout the life of the tires.

Also as noted above, data from the NCSA tire pressure survey indicate that about 29 percent of light trucks had at least one tire that was under-inflated by at least 25 percent. The average level of under-inflation of the four tires on light trucks with at least one tire under-inflated by at least 25 percent was 8.7 psi. Thus, on average, these light trucks could lose about 6,960 miles (8.7 psi under-inflation x 800 miles) of tread life due to under-inflation, if their tires were under-inflated to that extent throughout the life of the tires.

3. Reduced Fuel Economy

Under-inflation increases the rolling resistance of a vehicle's tires and, correspondingly, decreases the vehicle's fuel economy. According to a 1978 report, fuel efficiency is reduced by one percent for every 3.3 psi of under-inflation. ⁽²⁵⁾ More recent data provided by Goodyear indicate that fuel efficiency is reduced by one percent for every 2.96 psi of under-inflation. ⁽²⁶⁾

NHTSA notes that there is an apparent conflict between these data, which indicate that under-inflation increases rolling resistance and thus decreases fuel economy and the previously mentioned Goodyear data that indicates under-inflated tires increase a vehicle's stopping distance. While an under-inflated tire typically has a larger tread surface area (i.e., tire footprint) in contact with the road, which might be thought to improve its traction during braking, the larger tire footprint also reduces the tire load per unit area. The larger footprint does result in an increase in rolling resistance on dry road surfaces due to increased friction between the tire and the road surface. On dry road surfaces, though, the countervailing effects of a larger footprint and reduced load per unit of area nearly offset each other, with the result that the vehicle's stopping distance performance is only mildly affected by under-inflation on those surfaces. However, as explained above in section III.D.1., "Reduced Vehicle Safety -- Tire Failures and Increases in Stopping Distance," on wet surfaces other attributes of under-inflation lead to increased stopping distances.