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7 Myths About Saving Gas Busted (and 7 Tips That Really Do Work)

Does turning off the AC improve mileage? We separate fuel efficiency facts from fiction.

katie-conner
katie-conner
Katie Teague Writer II
Katie is a writer covering all things how-to at CNET, with a focus on Social Security and notable events. When she's not writing, she enjoys playing in golf scrambles, practicing yoga and spending time on the lake.
Expertise Personal Finance: Social Security and taxes
dan-avery-6315-1
dan-avery-6315-1
Dan Avery Former Writer
Dan was a writer on CNET's How-To and Thought Leadership teams. His byline has appeared in The New York Times, Newsweek, NBC News, Architectural Digest and elsewhere. He is a crossword junkie and is interested in the intersection of tech and marginalized communities.
Expertise Personal finance, government and policy, consumer affairs
Katie Teague
Dan Avery
6 min read
person pumping gas into vehicle

Can turning off the air conditioner mean you won't have to fuel up as often?

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Gas prices traditionally go up in the summer, as people hit the road for road trips and family vacations.

It's also the time of year when refineries shut down for maintenance, diminishing the available supply of oil. And hurricanes, tornadoes, wildfires and other natural disasters that can affect production and transport occur more often in warm-weather months.

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Even the fuel grade that retailers have to sell in summer is pricier.

There are countless tricks for saving on gas, from changing air filters to turning the car off at red lights. Some are legit, while others are a bust.

Below, we've lifted the hood on what gas-saving tips do and don't go the distance. 

For more, find out which credit cards have the best fuel rewards.

7 myths about gas mileage

Mechanic pouring additive into engine

The FTC has warned that most fuel additives have little or no impact on mileage.

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Skip these suggestions. They don't really work and may waste time and money.

1. Keep your tank full to prevent evaporation. Letting your gas gauge approach empty is never a good idea. But the theory that topping off your tank prevents fuel evaporation is bunk. Modern cars are all equipped with vapor recovery systems that minimize loss.

2. Buy gas early in the morning. The theory is that, because liquids become denser at cooler temperatures, you'll get more for your money by filling up in the a.m.  But gasoline is stored in underground tanks where the temperature is regulated. 

The roads may be less crowded in the morning, though, which could save you from wasting gas idling in traffic.

3. Replace your air filters often. Once again, this is a tip that may have worked once, but no more. Older cars filtered air into the carburetor, so a clogged filter could impact mileage. But today's engines have fuel injectors and other technology that carefully regulate the air-to-fuel ratio.

A study on gasoline engines conducted by Oak Ridge National Laboratory determined that "dirty engine air filters do not affect fuel economy in modern vehicles." Dirty air filters can, however, lead to sluggish acceleration and other issues. 

4. Overinflate your tires. If your buddy insists overinflating your tires will decrease resistance and save you on gas, tell them to hit the brakes. As little as 10 pounds per square inch over the manufacturer's recommended levels narrows the "contact patch," where the tire touches the road. That means less traction, a greater braking distance and more wear on the tires themselves, which ultimately eats up any negligible savings on gas. 

5. Change your oil frequently. It's a common misconception that if you're due for an oil change, your car's gas mileage will suffer. While it's best to keep up with maintenance, don't expect your mileage to improve after a trip to Jiffy Lube.

Your mileage may improve by up to 2%, however, if you use the manufacturer's recommended grade of motor oil.

6. Lower your tailgate. Pickup drivers like to claim that lowering your truck's tailgate is better for aerodynamics and thereby improves gas mileage. But according to General Motors' aerodynamics lab, up is better. 

"As air flows over the truck, it falls over the cab and pushes forward on the rear of the truck," the company said. "With the tailgate down, the benefits of that airflow are diminished."

7. Buy fuel additives or a "fuel economizer."  The Federal Trade Commission cautions consumers that claims by makers of devices and additives that promise to increase gas mileage "are either false or grossly exaggerated."

After testing more than 100 such products, the FTC didn't find any that significantly improved mileage.

Devices installed in your engine may even break the law by circumventing emission standards.

"People should be suspect of any device that promises to increase fuel efficiency," Patrick De Haan, head of petroleum analysis at Gas Buddy, told CNET.

7 gas-saving tips that really work

1. Slow and steady wins the race. Fast acceleration burns gas at a quicker rate than driving at a slower, more consistent pace. If you can maintain a constant speed, it will help you save gas. People who are constantly rushing and pressing on the gas pedal tend to consume more gas because of rapid acceleration, according to De Haan.

"If people drove at a slower pace of acceleration and avoided racing through a red light, it would help them prevent [...] burning through gas and using energy," he said. 

Vehicles are most efficient at between 55 and 60 miles per hour. Anything above that starts to deplete a car's efficiency, De Haan said.

2. Use cruise control when possible. The easiest way to maintain a constant speed? Cruise control. It's an easy way to maintain a constant speed, instead of slowing down and speeding up to get back to 55 miles per hour. It's best to use cruise control when you're driving on a flat road with no stops -- for instance, a highway. 

"Cruise control is much more effective than a human is at maintaining speed and can help save fuel," De Haan said.

3. Turn off your car at red lights and other long stops.  According to Linda Gaines, a transportation systems analyst for the Argonne National Laboratory, turn off your car if it'll be idling for 10 seconds or longer. Not only does it save on gas, but it reduces carbon dioxide emissions.

What about wear and tear on your starter?

"Today's starters are more robust than those in older cars," Gaines said. Unless you're revving up more than 10 times a day, "the starter motor is unlikely to need to be replaced during the vehicle's life."

She does discourage turning your car on and off in stop-and-go traffic, however.

"Driving safely means being able to respond quickly to traffic conditions," Gaines said.

4. Conserve air conditioning. According to the US Department of Energy, air conditioning use can reduce your fuel economy by more than 25%. But rolling down the window -- especially on the highway and at higher speeds -- can increase wind resistance and eat up more fuel.

De Haan recommends leaving the windows open if you're driving city streets, where you may be moving slower or idling more often. Otherwise, using the air conditioner is fine.

5. Take racks off your car when you're not using them. If you drive a vehicle that has large luggage racks on top, De Haan advises taking them off when you're not using them to boost your car's aerodynamics.

Smaller bicycle and ski racks are generally OK to leave on. 

6. Keep tires properly inflated. While overinflating your tires is no mileage hack, making sure they're properly inflated can improve gas mileage by up to 3%, according to the Energy Department.

Letting your tire pressure drop below 25 psi can cause an increase in friction, De Haan said, forcing the engine to work harder and getting you fewer miles per gallon. 

TPMS tire pressure light

Low tire pressure can mean fewer miles to the gallon. So keep an eye on your dashboard's tire pressure monitoring system light.

Patrick Whittemore/MediaNews Group/Boston Herald/Getty Images

Most cars will alert you when a tire is low. The tire pressure monitoring system, or TPMS light, typically looks like parentheses with an exclamation mark or dot in the middle.

7. Combine trips while running errands. If you have multiple errands to run on opposite sides of town, plan it so that you aren't driving back and forth. For instance, if the post office is next to the coffee shop but the post office isn't open yet, make that your last stop instead of having to drive back to that area.

When you do have to run errands at places that aren't nearby, it's best to try to do everything in one trip. Your car's engine is more efficient when it's warmed up, which can save you slightly on gas. It also prevents you from driving additional miles by taking trips on different days.