According to AAA studies, about 80 percent of the cars on the road are driving with one or more tires underinflated. Tires lose air through normal driving (especially after hitting pot holes or curbs), permeation and seasonal changes in temperature.
They can lose one or two psi (pounds per square inch) each month in the winter and even more in the summer, and you can't tell if they're properly inflated just by looking at them. You have to use a tire pressure gauge.
When tires are underinflated, the tread wears more quickly. According to Goodyear, this equates to 15 percent fewer miles you can drive on them for every 20 percent that they're underinflated.
Underinflated tires also overheat more quickly than properly inflated tires, which causes more tire damage.
Because of the extra resistance an underinflated tire has when it rolls, your car's engine has to work harder. AAA statistics show that tires that are underinflated by as little as 2 psi reduce fuel efficiency by 10 percent. Over a year of driving, that can amount to several hundred dollars in extra gas purchases.