Measurements of passenger car fuel consumption at different tire inflation pressures were recently conducted. These measurements were obtained from a vehicle operated on a test track using transient driving cycles and over a range of ambient conditions. The observed changes in fuel consumption, 0.33 percent/psi, should therefore be representative of actual driving conditions. Subsequently, tire inflation data from 2000 vehicles in six cites were analyzed to determine the extent of passenger car tire underinflation. These data sources were then combined to estimate the nationwide excess fuel consumption, about 600 million gallons per year, due to tire underinflation.