Under inflated tires: With the high prices of fuel, tire pressure becomes even more critical to the pocket book. The rounder the tire, the less energy it requires to roll. Tires that are even slightly below the recommended inflation pressure can reduce your mileage by up to 10 percent. If you do the math, that comes out to a lot of savings over the course of a year.
Almost three quarters of drivers have under inflated tires. Large swings in temperature over time reduces the pressure in tires.