You can improve your gas mileage by around 3.3 percent by keeping your tires inflated to the proper pressure. Under-inflated tires can lower gas mileage by 0.4 percent for every 1 psi drop in pressure of all four tires. Properly inflated tires are safer and last longer. (from fueleconomy.gov.)
by pb on July 25 2008
this tip works for 98% of voting Fuelly members.