How to Improve Gas Mileage: What Actually Works

Most fuel-saving advice online is either obvious or made up. Below are the changes the U.S. Department of Energy's FuelEconomy.gov site puts numbers on, roughly from biggest effect to smallest, and how to prove to yourself that they worked.

1. Drive smoothly

Hard acceleration, speeding and hard braking burn a lot of fuel. FuelEconomy.gov puts the loss from aggressive driving at roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic. For most people this is the biggest single change available, and it costs nothing.

In practice: accelerate gently, look further ahead so you can coast toward red lights instead of braking late, and keep a steady speed. Cruise control helps on the highway.

2. Slow down on the highway

Gas mileage usually drops quickly at speeds above about 50 mph. FuelEconomy.gov compares each 5 mph you drive over 50 to paying roughly an extra $0.29 per gallon (based on the fuel price it assumes, so the exact figure moves with prices). Driving 65 instead of 75 on a long trip makes a noticeable difference at the pump and adds only a few minutes.

3. Take the weight out

Every extra 100 pounds in your vehicle could reduce MPG by about 1%, according to FuelEconomy.gov. The effect is bigger in smaller vehicles. Tools, sports gear and boxes you've been meaning to drop off all count.

Roof racks and cargo boxes cost you even when empty, because they add drag at highway speed. Take them off when you're not using them.

4. Keep tires at the right pressure

Keeping tires inflated to the right pressure improves gas mileage by about 0.6% on average, and up to 3% in some cases. Mileage drops by about 0.2% for every 1 psi drop in the average pressure of all four tires. Under-inflated tires also wear out faster and are less safe.

Use the pressure on the sticker inside the driver's door, not the maximum printed on the tire. Check it when the tires are cold, about once a month and before long trips.

5. Don't idle for long

An idling engine gets zero miles per gallon. If you're parked and waiting, in a pickup line or outside a store, turn the engine off instead of letting it run.

6. Keep up with maintenance

A car that's overdue for service rarely gets its best mileage. Follow the maintenance schedule in your owner's manual and use the motor oil grade it recommends. A sudden, lasting drop in MPG is a common early sign that something needs attention. Keep a maintenance log so you can match changes in MPG to what was done.

Prove it with your own numbers

Pick one change, keep everything else the same, and compare your average MPG over three or four tanks before and after. Single tanks are too noisy: weather, traffic and fuel blends move them around. Here's how to calculate MPG accurately.

Milebook's fuel log does this for you. It charts MPG tank by tank, shows your average, and flags a tank that comes in more than 10% worse than usual. It also ranks the stations you use by the price you actually paid, so you can save at the pump as well as on the road.