How to improve gas mileage: what testing shows

Drive smoothly: the biggest lever

Hard acceleration and braking cost the most.

Fueleconomy.gov, run for the Department of Energy and the EPA, says aggressive driving — speeding, rapid acceleration, and hard braking — can lower gas mileage by roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic. At $4.47 a gallon, the site values driving sensibly at $0.45 to $1.79 per gallon. Looking ahead, easing off early, and coasting toward stops is the free version. The site also cites a study in which driver feedback devices improved the average driver’s fuel economy by about 3%.

Above 50 mph, every 5 mph costs more

Mileage usually falls quickly at highway speed.

Every vehicle has its own most efficient speed, but gas mileage usually decreases rapidly above 50 mph. Fueleconomy.gov estimates that each 5 mph over 50 is like paying an extra $0.31 a gallon at $4.47 gas, and that drivers willing to slow down by 5 to 10 mph gain 7% to 14%. Cruise control on the highway helps hold a steady speed and, in most cases, saves gas.

Drag and weight

The roof box matters more than the trunk.

A large, blunt rooftop cargo box can reduce fuel economy by about 2% to 8% in city driving, 6% to 17% on the highway, and 10% to 25% at 65 to 75 mph. A rear-mounted box or tray costs much less: 1% to 2% in the city and 1% to 5% on the highway. Take external carriers off when you are not using them. Weight matters less than drag: an extra 100 pounds reduces mileage by about 1%, and more in a small car than in a large one.

Idling gets zero miles per gallon

Turn the engine off when you are parked.

Idling burns a quarter to a half gallon of fuel per hour, depending on the engine and whether the air conditioning is running — about 2 to 5 cents a minute at $4.47 a gallon. Restarting takes only about 10 seconds’ worth of fuel, so switching off beats waiting with the engine running.

Maintenance that helps, and one that doesn’t

Tires and oil grade, yes; a new air filter, no.

• Tire pressure: keeping tires at the recommended pressure improves mileage by 0.6% on average and up to 3%; underinflation costs about 0.2% for every 1 psi drop in the average pressure of all four. Use the pressure on the door-jamb label, never the maximum printed on the sidewall.
• Motor oil: the manufacturer’s recommended grade improves mileage by 1% to 2%; using 10W-30 in an engine designed for 5W-30 can lower it by as much.
• Check engine light: a problem it reports can hurt fuel economy, so have the car inspected when it comes on.
• Air filter: on fuel-injected, computer-controlled engines — most cars built since the early 1980s — replacing a clogged filter improves acceleration but not fuel economy.

The fixes, ranked

Largest effect first, per fueleconomy.gov.

Effects vary by vehicle. To price a particular trip, use the fuel cost calculator; to compare electricity, see what it costs to charge at home.