Better gas mileage, and what actually moves it

How you drive and how you keep the car both affect gas mileage. Some habits make a big difference and others make very little.

How you drive

Aggressive driving costs the most. Speeding, fast starts and hard braking can lower gas mileage by roughly 15 to 30 percent at highway speeds, and by 10 to 40 percent in stop-and-go traffic.

Idling burns fuel while the car goes nowhere, about a quarter to a half gallon an hour depending on the engine and whether the air conditioning is on. Turn the engine off if you'll be parked for more than a short wait. On the highway, cruise control helps you hold a steady speed and in most cases saves gas.

What the car carries

An extra 100 pounds in the car can cut mileage by about 1 percent. A roof cargo box costs much more, around 2 to 8 percent in city driving and 6 to 17 percent on the highway. A cargo box or tray mounted at the back of the car costs far less.

How the car is kept

• Tire pressure. Tires at the right pressure improve gas mileage by 0.6 percent on average, and up to 3 percent in some cases. Every 1 psi drop in average pressure lowers mileage by about 0.2 percent. The right pressure is on a sticker in the driver's door jamb or the glove box.
• Motor oil. Using the grade the maker recommends can improve mileage by 1 to 2 percent.
• Air filter. On a modern fuel-injected engine, replacing a clogged air filter doesn't improve gas mileage, though it can improve acceleration.

An annotated example

What to do next

Check the tire pressure against the door-jamb sticker this month. Look up the oil grade in the owner's manual and make sure the shop uses it. Take the roof box off between trips, and clear out heavy things that ride along for no reason.