A few car maintenance myths and the money-saving truth.

Myth: Air conditioning will hurt fuel economy

Reality: There has been much debate about whether to drive with the air conditioner on or keep the windows open in order to save gas. Using the A/C does put more load on the engine, but in our tests, we found just a slight decrease in fuel economy and no measurable difference when opening the windows (open windows do increase aerodynamic drag). However, using the A/C helps keep the driver alert and more comfortable, which is safer for everyone on the road. We say, just use the A/C and don’t worry about it.

Myths: You’ll get more gas for your money if you fill up in the morning.

Reality: 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. However, the temperature of the gasoline coming out of the nozzle changes very little, if at all, during any 24-hour period of time since it is stored in underground tanks. So long as the gas station does even a modest business, gas won’t heat up much in the pump, and even if so, that is a relatively small amount. For the extra effort to chase cool temperatures, any extra gas you get will be negligible, and making a special trip will certainly burn far more than it would be possible to save. Just buy when it is convenient.

Myth: Inflate tires to the pressure shown on the tire’s sidewall.

Reality: The pounds-per-square-inch figure on the side of the tire is the maximum pressure that the tire can safely hold, not the automaker’s recommended pressure, which provides the best balance of braking, handling, gas mileage, and ride comfort. That figure is usually found on a doorjamb sticker, in the glove box, or on the fuel-filler door. If the tire pressure is down 10 psi, our testing has shown that it can make a 1 mpg difference in fuel economy. But far more significant is the impact on handling, braking, and wear–all of which can cost you one way or another. Check the tire pressure monthly after the car has been parked for a few hours.

Myth: Premium gas is better for your car.

Reality: Most vehicles run just fine on regular-grade (87 octane) fuel. Using premium in these cars won’t hurt, but it won’t improve performance, either. A higher-octane number simply means that the fuel is less prone to pre-ignition problems, so it’s often specified for hotter running, high-compression engines. So if your car is designed for 87-octane fuel, don’t waste money on premium and if you car recommends (not requires) premium, you can usually get away with using regular. Some cars truly require premium, meaning you’re stuck paying extra. Keep this in mind when shopping for your next car.

Copyright © 2007-2013 Consumers Union of U.S., Inc.


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