You can waste a lot of breath grousing about high gas prices. You can drive 10 minutes out of your way for every fill-up just to save a couple of cents per gallon. Or you can invest your time and energy where it really counts-in your car. Instead of complaining, take a hard look at four areas of upkeep where you can reap the greatest rewards in fuel efficiency:

Tires. Far and away, underinflated tires rank as the most common reason for wasting gas. According to reliable on-the-road surveys, one out of every four cars has at least one extremely low tire (below specs by 8 psi or more). One in every three pickups, vans, and SUVs has the same problem. Gas mileage drops as much as 2 percent for every single pound of pressure your tires fall below the recommended level. Investing five minutes with a tire gauge and an air hose can save a bundle.

Engine. Neglected tires may be the most common cause of poor mileage, but out-of-tune engines rank as the most costly. Old, dirty spark plugs can reduce fuel economy by 30 percent. Other bad components that cause your engine to run out-of-spec can waste even more. An idle speed set too high, for example, burns extra gas every time you wait at a red light. Check your owner's manual for required engine maintenance internals.

Air filter. Think of the air filter as the "most forgotten" way to save gas. Many motorists simply don't remember to change it every two years. But a dirty air filter can effectively add several pennies per gallon to the price of gas at every fill-up.

Peripherals. Systems that you don't normally think of as related to fuel economy can significantly affect gas mileage. For instance, a defective thermostat (part of the cooling system) can cause longer warm-ups and sub-optimal operating temperatures-thereby decreasing your mpg. And a bad thermostat may show up first as a heater that blows little or no hot air. Track down little problems before they balloon into big ones; the savings at the pump may surprise you.