Better than the penny test, check to see if the treadwear is at or past the "wear bars": These bars are designed into most road tires to indicate the manufacturer's marker for when the tire should be replaced due to normal treadwear.
An underinflated tire flattens out, perhaps even riding on the thin unprotected sidewall. More tire surface area means greater road friction. Proper tire care can improve handling, tread life and fuel economy. Your vehicle loses 10 percent in fuel economy on average for every five to seven pounds reduction in air pressure, Zielinski said.