An informal study by students at Carnegie Mellon University found that the majority of cars on U.S. roads are operating on tires inflated to only 80 percent of capacity. According to the website, fueleconomy.gov, inflating tires to their proper pressure can improve mileage by about 3.3 percent, whereas leaving them under-inflated can lower mileage by 0.4 percent for every one PSI drop in pressure of all four tires.