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Debunking fuel-economy myths

Get a reality check on 7 common fuel-saving misconceptions

Consumer Reports magazine: August 2012

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. (Learn more about fuel economy and how to save at the pump.)

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.

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.

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.

Independent stations usually buy their fuel from larger, name-brand oil companies, so it’s not much different from what you’d get for a higher price down the road. Off-brand gasoline is sometimes formulated without additives designed to clean the engine, but your car should run fine on that gas.

When it comes to regular, midgrade, and premium gasoline, oil corporations have worked overtime to drill the “good, better, best” concept into our collective driver psyche.

Premium gas has a higher octane rating, usually 91 or above, making it more resistant to pre­ignition, a condition in which fuel burns uncontrollably in the engine. Higher-performing engines are the most susceptible to preignition because they tend to run hotter, which is why premium is often recommended or required for sports and luxury vehicles.

Premium also helps maximize power in high-performance engines. With those engines, if you don't use premium, you might not get full power when, say, accelerating or climbing hills. Most drivers will probably never notice the difference.

The vast majority of cars are designed to run fine on regular. And premium won’t improve performance or fuel economy for those cars, but it will cost you about 20 cents more per gallon.

Our advice: The best gas for your car depends on the vehicle you drive. If the owner’s manual or the sticker on the fuel-filler door says that premium gas is recommended or uses similar wording, you can probably use regular. If it says premium is required, play it safe with the right octane.

Some people advise you not to run the air conditioner because it puts more of a load on the engine, which can decrease fuel economy. But others say that opening the windows at highway speeds can affect gas mileage even more by disrupting the vehicle's aerodynamics. In our tests of a Honda Accord, using air conditioning while driving at 65 mph reduced the vehicle's gas mileage by more than 3 mpg. The effect of opening the windows at 65 mph was not measurable.

A lot of attention is paid to a tire’s rolling resistance, which is how much energy it takes to roll along. The lower the rolling resistance, the better your fuel economy will be. Maintaining the proper tire pressure will optimize the rolling resistance and real-world performance. Some tires gain low rolling resistance at the expense of wet-braking performance and tread life—a poor trade-off.

It’s better to look first for a tire that provides good all-around performance in important safety areas such as braking, handling, and hydroplaning resistance. Then use rolling resistance as the tiebreaker.

In our testing, we’ve found two all-season passenger-­car tires that deliver very good performance and low rolling resistance: the Continental ProContact EcoPlus+ and the Michelin Energy Saver A/S. (See our tire-buying advice and Ratings.)

Editor's Note: A version of this article appeared in the August 2012 issue of Consumer Reports magazine with the headline "Myths at the Gas Pump."
   

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