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) -- When learning to drive, teens get to hear some old adages and "rules" from Mom and Dad that resonate with them for years. With 39 million Americans planning to

hit the road for Fourth of July weekend

, MainStreet decided to play driving detective to find out whether certain pieces of car advice passed down from parent to child still hold true. As it turns out, some pieces of conventional knowledge still hold up, but today's drivers could stand to check their owners' manuals a little more often.

We gave each adage a verdict of "myth" or "rule," stating that there are some exceptions to certain items. Put your cursor on cruise control as MainStreet takes on our favorite driving myths and facts, including whether it really pays to pump your gas at the crack of dawn and how much traction you'll actually get by overfilling your tires. Get ready to start your engines...

Change the Oil Every 3,000 Miles

Verdict: MYTH

Salty car mechanics and Jiffy Lube aren't the only know-it-alls warning drivers to change their oil every 3,000 miles or else, but are modern cars really that needy? Not necessarily, says Christie Hyde, a spokesperson for AAA.

"Most modern cars can run 5,700 to 10,000 or more miles before you need to change your oil," she says, "but it's been ingrained in people's heads, so

AAA always advises drivers to pull out the owners' manual and check the maintenance schedule to know for sure."

Thanks to new types of synthetic motor oils and innovations to car engines themselves, how frequently you need to change the oil has changed over time, says Hyde. Just keep in mind that the harder your car has to work, the more upkeep it's going to need.

"If you're mostly in traffic at low speeds -- think severe conditions, dusty and rough terrain -- you might want to change the oil more often," Hyde says.

Put Sandbags in the Trunk for More Traction

Verdict: MYTH (except for rear-wheel drive vehicles)

Blame the icy roads in winter for compelling Dad to make us drive around with a bunch of kitty litter in our car trunk.

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"People used to put sandbags in the trunk of the car to increase traction," says Phil Reed, senior consumer advice editor for

, a car review site. "The myth is true unless your car is front-wheel drive. But in most cases, it's probably no longer true because the design of cars have changed quite a bit since the '70s when the car engine started to be more commonly placed directly over the wheels. It's a less complex, more direct design," plus it helps with handling, too.

Overfill Your Tires in Winter to Boost Traction

Verdict: MYTH

Drivers in cooler regions like the Northeast and Midwest can face some treacherous conditions on the road, especially in winter. So does overfilling one's SUV tires make any difference in how they grip the pavement? Reed isn't so sure.

"If the tires are very soft, the car won't handle well and will be unsafe." But if you fill up past the recommended tire pressure, Reed says "only the center will be touching the road, which could adversely affect the handling."

The idea is to hit what Reed calls a "sweet spot": You want to inflate the tire to the right PSI, or pounds per square inch, level in order to get a small gain in your fuel economy and still be safe. "I wouldn't go more than four to five over the recommended PSI level. Check the owner's manual and, or look for the yellow sticker in the door jamb

which lists tire inflation guidelines on the driver's side."

"Many modern cars have tire pressure monitoring systems so the level shows up on the gauge on the dashboard," Reed adds, making it easier for drivers to keep track of all that hot air.

You Can Keep Driving for a While on an Empty Tank

Verdict: RULE

Can you really go a long distance on empty? Well, that depends on the car manufacturer, says Reed. "Some manufacturers are extremely cautious, like Toyota; the Toyota Prius will warn you when it still has two to three gallons

left in the tank, which is called being nannyish," he says. The real answer can be found in your car manual, or by trying Reed's clever trick.

"Look in your manual and find out the size of your gas tank. If it holds 15 gallons, for example, wait until the fuel gauge reads empty, then fill it up. If it takes 12 gallons to fill it up, how many were left?" That's how you'll know what "empty" really means -- 3 gallons, in this case, which means you could drive as many as 75 more miles if the car gets 25 miles per gallon.

"It's about learning your car," Reed says.

Premium Gas Is Always Better Than Regular

Verdict: MYTH

Back in the day, buying premium gas was imperative, says Reed.

"If you didn't use premium, sometimes the car would ting or knock. The engine would make a funny noise, called pre-ignition, and that meant that the cheap gas was exploding on its own or before the sparkplug had fired," he says.

But thanks to modern technology, starting around 1990, car manufacturers started equipping their engines with knock sensors that send info to the engine's operating system to tell the car's engine to adjust the timing to stop it, or deal with the issue.

"There's a small number of cars that still require premium gas," says Reed. But even if pricey gas is called for, drivers can still try using a lower grade gas to see how their car performs, Reed says. "I predict your car won't see a difference; it can probably go to a lower gas."

Fill 'er Up at the Crack of Dawn

Verdict: MYTH

Gas prices this summer are enough to make drivers' heads spin. So we can't blame them for looking for slick ways to save, even if that means setting the alarm for 4 a.m. to wake up and fill 'er up. But is all that effort worth it? Reed, the expert, says to hit the snooze button.

"The reasoning is that at colder temps fluids are more dense," he explains. "So theoretically, if you pumped cold gas, you would get more. But gasoline is stored underground, where the temperature is constant, or nearly constant. In the quantities that we deal with, it would be the difference of even one penny, maybe a fraction in some extreme cases. Do you really want to get up in the morning in the off chance you'll save a penny?"

As Reed laments, consumers "are looking in the wrong direction." The real money saver, he says, is driver behavior.

"Aggressive driving -- rapid acceleration, high cruising speed, high braking speed -- all those things are wasteful in three ways," he says. "People don't like slowing down, but what you need to do is drive cool and smart, and you'll save 30%. It's free and you can start doing it today. You won't wear out your car and you won't be stressed."

Idling Saves More Gas Than Turning the Engine Off and On

Verdict: MYTH

Jack Nerad, an executive analyst for the car review site

Kelley Blue Book

, advises consumers against idling, period.

"Turning it off is better for fuel and it's safer," he says. "With today's cooling systems, it's OK to let the car idle, but it's better for fuel economy not to."

The same goes for stepping out of the car to take a quick stretch. If you get out of your car, that won't affect gas usage, Nerad says.

"What's happening is the engine's burning the fuel at the same rate, whether you're in or out of the car. If you're going to get out of the car, turn the car off."

Always Warm Up Your Car Before You Start Driving

Verdict: MYTH

"Warming up the car for five minutes in front of the house is unnecessary," says Nerad. "You don't want to drive it aggressively at first, to get the parts working with each other rather than against each other. It's just not good to drive aggressively or 'gas it.'"

And as for warming up the car in wintertime, Nerad adds that it's "necessary for your butt, probably not for the car. Most people do it to get the interior up to a reasonable, more comfortable level. When the car is not warm, you can warm it up by driving it easily for the first mile or two."

You Can Drive Perfectly Well on a Spare Tire

Verdict: MYTH

How long can you rely on a spare after a tire blowout?

"There are two issues," says Nerad. "First, the longevity of the spare, and second, the handling or lack therof."

In other words, if the tire's built to last, that's one thing, but if the shape seems to differ from that of your other tires, you're going to have a serious driving conundrum on your hands. Weird handling is a recipe for accidents or worse, so "you'd want to get that off pretty quick," Nerad says. Again, check your owner's manual: most advise drivers to ride on the spare for 50 miles or less.

Written by Jill Krasny of MainStreet.

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