Myth Buster: More Bunk About Better Gas Mileage

Myth: You get better mileage if you fuel up in the morning.

Facts: This doesn’t check out. The misconception goes something like this: In cooler morning air, gas is denser. If you pump gas when it’s cooler, it will expand and give you more than you paid for.

It’s technically true that gas expands at warmer temperatures, says Jeff Bartlett, deputy autos editor at ConsumerReports.org. But the real-world impact is minimal. And because gas is stored in insulated tanks underground that maintain stable moderate temperatures, he says, the temperature of the gas passing through the pump varies very little. Even when the gas is in your tank, the expansion caused by increasing temperatures throughout the day will be negligible.

Myth: Additives to gasolines improve your car’s fuel efficiency.

Facts: “We have tested many different types of gas additives, and we have never found any that did anything to improve fuel efficiency,” says Steve Mazor, manager of the Automobile Club of Southern California’s automotive research center. Likewise, of more than 100 different fuel additives tested by the U.S. Environmental Protection Agency, none was found to improve a car’s gas mileage.

Myth: Premium gas gets better mileage than regular.

Facts: This myth goes back to the 1960s, Mazor says, when only premium gas had detergent, which helps clean the engine and improves its efficiency. However, now all fuel grades have detergents.

“Our recommendation is to read your owner’s manual,” says Mazor. “Ninety percent of cars on the road run on regular gasoline.” If you use premium when it isn’t required, he says, the only thing you’re draining is your wallet.