Driving Away Mileage Myths

The price of gas has soared more than 60 percent since December. With that in mind, more people are looking to maximize mileage this Fourth of July weekend.

But there are lots of mileage myths out there. And Jill Schlesinger, editor at large of CBS Moneywatch.com, separated fact from fiction on "The Early Show Thursday:

1. MYTH: If your owner's manual says "premium fuel recommended," you'll ruin your car by filling it up with regular.

REALITY: Unless you're driving in the Indy 500, using regular gas in a car that says premium fuel is merely "recommended" is perfectly fine. And doing so will save you 25 cents per gallon.

Gas prices are rising again.

"Unless you have a job that requires regular sessions on a racetrack, there is no legitimate need to ever put premium fuel in your vehicle," said Susan Winlaw, co-author of the book, "Car Advice for Women (and Smart Men)." Using regular gas could cost you a few horsepower when you're driving at higher speeds, according to Winlaw, but chances are you won't notice the difference, and it definitely won't hurt your car. Your engine's fuel-management system is perfectly prepared to handle lower-octane fuel, said Winlaw.

On the other hand, switching to regular in a car for which premium fuel is "required," as it is for a few high-performance luxury rides, could cause noticeable knocking. And over time, that could lead to faster engine wear.

Those cases aside, just how much could switching to regular gas save you this weekend?

According to the AAA's Daily Fuel Gauge Report, the national average price per gallon recently was $2.68 for regular gas, versus $2.94 for premium. A long road trip could account for four fill-ups of about 20 gallons each, meaning you'd save almost $21 by using regular gas over premium.

2. MYTH: In the summer, you should only buy gas at night or early in the morning when the gas is cold. Because cold gas is denser, you'll get more fuel for your money.

REALITY: Buying cold gas is a lot harder than it sounds, and the potential savings are scarcely worth it.

Consumer Reports thoroughly tested this theory using their own underground tank, similar to those used by gas stations. They found that it's surprisingly hard to accurately predict whether a given tankful of gas will be cold or warm. For starters, if gas was a given temperature when it was delivered from the tanker truck, it tended to stay that temperature for a while, even after it had been transferred. Not only that, but the first gas to be pumped in a given day could be warm because a certain amount of gas collects in the aboveground pump. So even if you manage to be the first customer of the day, you might still be buying warm gas.

After all that, even if you're successful in buying cold gas, the difference in density is so slight -- perhaps a maximum of one percent per fill-up, according to Consumer Reports -- that the savings are marginal.

"It's an urban myth" that you should always buy gas at night or early in the day, Gabriel Shenhar, senior auto test engineer for Consumer Reports.

3. MYTH: Shopping around for cheaper gas is seldom worth the extra fuel you burn up trying to find it.

REALITY: It's easy to find where the cheapest gas stations are online, and crossing state lines when you can saves plenty of money.

The AAA's TripTik Travel Planner not only gives you point-to-point driving directions, but also allows you to highlight gas stations along your route, including frequently updated gas prices for each location. You'll have to zoom in to a pretty detailed view of your route, but the tool allows you to plan where to stop for gas ahead of time instead of roaming around looking for good prices. You don't even have to be an AAA member to use the feature.

And especially on an interstate road trip, planning ahead can save you a significant amount of money because of different state taxes on gas. Some states with the lowest average cost per gallon are next door to some of the highest-cost states, such as New Jersey and New York. The recent New Jersey average price per gallon was $2.56 for regular versus $2.85 in New York, according to the AAA. If you're already planning to cross the George Washington Bridge, then, you should plan to buy gas on the New Jersey side, where a full tank will cost almost $6 less, which will almost help pay the $8 toll for the bridge.

4. MYTH: For the best gas mileage, you should keep it below 55 miles per hour.

REALITY: Fuel efficiency doesn't really start to drop until you reach speeds higher than 60. And how smoothly you drive makes much more of a difference on gas mileage than how fast.

According to the U.S. Department of Energy (DOE), most cars' fuel efficiency peaks at speeds from 35 to 60 miles per hour. After 60, though, fuel efficiency drops significantly; the DOE says that every five miles per hour you drive above 60 is like paying an additional 24 cents per gallon for gas. That's because at higher speeds your car encounters more wind resistance and the tires encounter more rolling resistance.

Once you get your car going, though, it takes remarkably little energy to keep it going, even at 60 mph, according to Terry Penney, technology manager for advanced vehicles for the DOE's National Renewable Energy Laboratory in Golden, Colo. But getting it rolling burns up a lot of gas, which is why you should listen to your old driver's-ed teacher and drive as if there's an egg between your foot and the gas pedal. The DOE estimates that constantly accelerating and decelerating can cut your mileage by as much as 33 percent. The fact is, easing up on jackrabbit starts and Indy-500-style highway maneuvers will save you more money than puttering along at 55.

5. MYTH: A car with manual transmission will always get better mileage than one with an automatic.

REALITY: Newer automatic transmissions can get the same highway mileage as manual transmission (or even slightly better).

Following previous oil embargoes, there was a rush on cars with manual transmissions because they got better mileage than automatics. But that's less true today, especially at highway speeds, says Gabriel Shenhar, senior auto test engineer for Consumer Reports. Older three-speed automatics had to work a lot harder at highway speeds than today's four- or five-speed automatics. And the new automatics produced in the last few years have an "overdrive" top gear that reduces engine rpm's at higher speeds.

In city driving or mixed city-highway driving, a manual still tends to get better mileage, Shenhar says. But on the highway and long driving trips in particular, your automatic may get as good or even slightly better mileage than the manual. The 2009 Chevrolet Aveo, for instance, gets an EPA-estimated 34 miles per gallon highway. And most versions of the sporty 2009 Mazda3 get 29 miles per gallon on the highway, manual or automatic.

6. MYTH: To save gas, open the car's windows and shut off the air conditioning.

REALITY: At higher speeds, leaving the windows open increases wind resistance so much that you probably wipe out any gain from shutting off the air-conditioning.

Air conditioning does burn up gas, which is why Ford recommends that you at least turn down the AC and use the "vent" settings on your climate control as much as possible. According to Ford's website, Driving Skills for Life, reducing AC usage when the temperature is above 80 degrees can save up to 10 to 15 percent on fuel.

But at speeds above 50 miles per hour, lowering the windows increases wind resistance so much that you're better off closing them and putting on the AC at a moderate temperature. It's also good to remember that once a car is cool inside, it takes a lot less energy to keep it cool. Result: Some of the best AC-related mileage boosting you can do occurs when the car's not moving. That is, when it's hot, park in the shade.

AAA also projected the number of Americans traveling on vacation this holiday weekend will decrease 1.9 percent from 2008 with approximately 37.1 million travelers taking a trip of 50 or more miles away from home.

Last year, AAA reported, 37.8 million Americans traveled during the same period; a 10.5 percent decline from the 42.3 million travelers who vacationed in 2007.

AAA said the projected decline in leisure travel is mainly due to ongoing uncertainty about the strength of the economy; especially rising joblessness and sagging personal incomes. The recent rise in gasoline prices may also be causing some travelers to limit or abbreviate their holiday plans, although its impact on overall vacation costs remains minimal, AAA said.