Under-inflated tires are one of the most commonly ignored causes of crummy MPG. Tires lose air due to time (about 1 psi per month) and temperature (1 psi for every 10 degree drop); under-inflated tires have more rolling resistance, which means you need to burn more gas to keep your car moving. Buy a reliable tire gauge and check your tires at least once a month. Be sure to check them when they are cold, since driving the car warms up the tires along with the air inside them, which increases pressure and gives a falsely high reading. Use the inflation pressures shown in the owner's manual or on the data plate in the driver's door jamb.
