Fact or Fiction? Gas Myths Debunked

June 10, 2008 — -- Roll down your windows and turn off the A/C. Use premium gas. Fill up your tank early in the day.

As cost-conscious drivers gather around the ever-gouging gas pump, these are just a few of the oft-repeated tenets of fuel-saving wisdom. But according to experts, many of these so-called tips are mere myths. Gearheads from the American Automobile Association, Consumer Reports and "Mythbusters" helped us sort fact from fiction.

Your Friends Say: Air Conditioning Drains Your Gas Tank

Experts Say: It's So Insignificant, You Might as Well Be Comfortable

One of the most commonly-heard myths touted by consumers and Manhattan cab drivers alike is that running the A/C drains your gas tank. Not true, according to Michael Calkins, the manager of approved auto repair at AAA — at least not on the highway.

According to Calkins, on the highway, both air conditioning and open windows affect fuel economy in similar ways: only about half a mile per gallon.

"In highway speeds, [open windows] do increase your drag," Calkins said.

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New York cab drivers aren't completely wrong. In stop-and-go traffic, open windows don't have nearly as much drag, so open windows may rob less fuel than the air conditioner.

"A/C uses slightly more gas, but the savings are not really significant," Caulkins said. "But when you consider the fatigue factor, it's better to use the air conditioning though it may cost you half a mile a gallon. — If ultimate fuel efficiency is your only goal, then driving with your windows down will save you a tiny bit of fuel around town certainly, but not on the highway."

Your Friends Say: Buy Gas Early in the Day

Experts Say: Buy Gas When You Need It

Another common gas pump mystery surrounds the time of day to buy gas. The thinking goes something like this: As it gets hot, gas expands in the tank, so if you buy early in the day, you'll get more bang for your buck.

But tailoring your gas-buying habits to this idea isn't particularly useful, according to David Champion, director of automobile testing at Consumer Reports.

"The hotter the day … the more gas will evaporate as you put it into the car. When your car is nice and cool, there's less evaporation as you put it into the car," Champion said. "It doesn't make much of a difference to the individual consumer. … It's 1 percent [of gas] a year that you may use."

According to Calkins, no matter what time of day you buy it, gas is generally always going to be the same temperature.

"[Most] gas is stored in underground tanks," he said. "It doesn't change in temperature at all."

Your Friends Say: Premium Gas Gets Better Gas Mileage

Experts Say: If Your Car Can Take a Lower Grade, Go For It and Save

While AAA's Calkins generally advises people to follow their owner's manual, this is one area where in regular driving conditions he says that drivers can deviate.

"The reality is manufacturers' fuel recommendations are based on the performance potential of the engine," he said. "On modern cars, even when it says to use premium fuel, mid-grade or regular fuel will work. It will reduce your performance, but if you're not going to be towing a trailer or racing, you don't necessarily need 100 percent of the power."

Calkins says that as long as you don't hear a knocking sound, you have no problem.

"If you do hear that, you should immediately go back to a higher grade," he said.

Your Friends Say: Clean Air Filter = Better Gas Mileage

Experts Say: Pay Attention to Tires, Not Air Filters

There are a lot of things that can affect your fuel economy – the weight of your car (ditch those roof racks, people!), for example, but a dirty air filter isn't one of them.

According to Champion, if the car was made in 1997 or later, the engine automatically adjusts for a dirty air filter.

"The car feels sluggish, but in terms of fuel economy, it doesn't make any difference," he said. "In older cars, it will make a difference."

Another thing that makes a difference? Underinflated tires.

Champion recommends following the recommendations of your vehicle's manufacturer and checking your tire pressure once a month.

"If your tires are underinflated, it will use more gas," he said. "It's a bit like riding a bicycle. With deflated tires, it's really difficult to pedal. That's exactly how it works on the car."

Your Friends Say: Use the 10-Second Rule

Experts Say: Use the 30-Second Rule

A lot of credence has been given to not letting your car idle and instead stopping it and restarting it to save gas.

According to Champion, no one should stop their car in regular traffic just because they may be idling for a few seconds.

"Basically, if you think you're going to be stopped for more than 30 seconds, turn your engine off. Although an idle doesn't use very much fuel at all," he said.

Your Friends Say: Try Additives

Experts Say: 'Fuel Economy' Gadgets, Potions Are Bunk

No matter what you see in the back ads of "Popular Mechanics," don't buy any additive or gadget that claims to increase gas mileage, experts say.

"I can sum up all of these devices in one sentence: None of them work," Calkins said. "The EPA has a complete test procedure for anything that claims it increases fuel economy. To my knowledge, not one single manufacturer has ever successfully completed that protocol and been able to show scientifically [it works]. As much as we all want to believe in the tooth fairy and the Easter bunny, you can't buy them on the shelf at the auto parts store."

Champion concurred, saying the magazine has tested many of these devices and never found one that worked.

Your Friends Say: Go for a Drive to Chill Out

Experts Say: Stress Reduces Your Fuel Economy by 50 Percent

"Mythbusters" co-host Adam Savage has made a living debunking gas-saving myths on his popular Discovery channel show. The only thing he's found that really saves gas? Relaxing. In an experiment that will air in an upcoming show, the "Mythbusters" team found that people who drive while they're angry use 50 percent more gas than people who are relaxed.

"The difference was absolutely astonishing," Savage said. "You just use a lot more gas [when you're angry]. That has more of an effect on your fuel economy than any other single thing."

There's no magical trick to saving gas, the experts told ABCNews.com; it's all common sense.

"Drive as if you have a cup of coffee on your dashboard," Champion said. Smooth acceleration and deceleration improve fuel economy as does driving while your car is "warm."

"Put all your trips together," Calkins said. "Cars use a lot of fuel when they're cold."