Debunking 7 fuel-economy myths


Like all drivers, you want to save gas and do what’s right for your car. But along with the tried-and-true advice, there are some well-intentioned—if off-the-mark—tips that can lead you astray. Below are several common myths about fuel use and gas mileage, and the real stories behind them.

A dirty air filter drops gas mileage

Our tests show that driving with a dirty air filter no longer has an impact on fuel economy, as it did with older engines. That's because modern engines use computers to precisely control the air/fuel ratio, depending on the amount of air coming in through the filter. Reducing airflow causes the engine to automatically reduce the amount of fuel being used. Fuel economy didn't change in the family sedan we tested, but it accelerated much more slowly with a dirty filter.

Warming up before driving is necessary

That was true back in the days of carburetors and chokes, but it isn’t the case with modern fuel-injected, electronically controlled drivetrains. Engines are most efficient when they’re at regular operating temperature, and the fastest way to reach that point is to drive right after starting the car.

Filling up when the air is cool gets you more gas

A common tip is to buy gasoline in the morning, when the air is cool, rather than in the heat of the day. The theory is that the cooler gasoline will be denser, so you will get more for your money. But most stations store the gasoline underground, so its temperature changes very little, if at all, during a 24-hour stretch. Any extra gas you get will be negligible.