What if a common car myth that you thought to be true was costing you time and money?

It wouldn’t be surprising. With the exception of smart mechanics and car enthusiasts, most of us take car knowledge for granted. We assume that the tips and advice we hear -- no matter where we heard them -- are true.

But that’s not always the case. In fact, some of the most widely accepted beliefs about cars are actually myths, and costly ones at that. Before you’re duped any longer, check out the following list of common car myths and learn why they’re simply not true.

Myth 1: You should change your motor oil every 3,000 miles.

If you don’t change your motor oil on a regular basis, your engine will go kaput. However, it doesn’t need to be done every 3,000 miles. In fact, most new cars are engineered to drive more than double that distance without running your engine dry.

“Because there are many factors at work -- how you drive, the condition and age of the engine, the external environment you drive in, and stop-and-go versus highway driving -- it's an inexact science,” write the pros at Car Talk. Still, they recommend “that you change your oil and filter every 5,000 miles.  … It may be too soon for many people and too late for a few, but for the vast majority, 5,000-mile oil changes will help your engine last to a ripe, old age.” Better yet, check your owner’s manual for the exact mileage point at which you should have it changed.

With the average passenger vehicle traveling anywhere between 12,000 and 15,000 miles per year, having your oil changed at the 5,000-mile mark can reduce the number of annual visits you make to the lube shop from four or five to two or three. That means less time in the waiting room and more money in your pocket.

Myth 2: Filling your tank with premium gasoline will make your car run better.

If your car is designed to run on regular gasoline, you shouldn’t bother filling it with premium. “In fact, in most cases, using a higher octane gasoline than your owner's manual recommends offers absolutely no benefit,” reports the Federal Trade Commission. “It won't make your car perform better, go faster, get better mileage or run cleaner. Your best bet: listen to your owner's manual.”

In some cases, however, you may want to experiment with premium-grade gasoline. For instance, if your engine makes an unusual knocking sound, filling with premium may make it stop. And if your owner’s manual says that your car requires it, don’t ever downgrade to regular. Otherwise, premium gasoline is just a waste of money. “Studies indicate that altogether, drivers may be spending hundreds of millions of dollars each year for higher octane gas than they need,” says the FTC.

Myth 3: You need to warm up your engine before taking off.

In the past, cars were equipped with carbureted engines that needed to be started and left to idle before driving away. However, today’s cars feature fuel-injection systems that don’t require that step. “As far as warming up an engine before driving away, that old nugget no longer applies because modern engines have better lubrication, tighter clearances and, in general, are more resistant to sludge formation,” writes Popular Mechanics. “Start the motor, buckle your seatbelt, adjust the mirrors and drive off at normal speeds.”

These days, leaving your car to idle can be costly. Not only is it a waste of gas, but if left unattended your car is susceptible to theft. It also puts unnecessary wear on your engine and releases harmful emissions into the environment. Besides, who has time to just sit and wait in the driveway?

Myth 4: Brand-new cars are stolen more often than older cars.

Actually, car thieves prefer old vehicles over new ones. In fact, the National Insurance Crime Bureau’s (NICB) 2009 “Hot Wheels” report shows that the most commonly stolen vehicles are five to 10 years old -- with the 1994 Honda Accord topping the list. These cars are targeted because their parts are worth a lot on the black market, but also because older vehicles are less likely to be equipped with the latest anti-theft technology -- making them easy for criminals to nab.

All in all, car theft has hit a 20-year low in the United States. But that doesn’t mean your car is safe. If you are going to buy an older model, spring for an engine immobilizer, tracking device or even a steering wheel lock. Also consider buying more comprehensive auto insurance coverage. Or, spring for a new car and deter would-be car thieves altogether.

Myth 5: You’ll save money by driving a hybrid car.

Yes and no. Hybrid cars generally cost more than their traditional gasoline-powered counterparts. So opting for a hybrid powertrain only makes good financial sense if the amount of money you’ll be saving in gas is more than the premium you’ll pay to buy the hybrid -- over a reasonable amount of time, of course.

Don’t fret; the math is actually quite simple. Divide the average number of miles per year that you drive by the vehicle’s combined fuel economy rating, and multiply the answer by the national average price of one gallon of gasoline. Doing this reveals how much it will cost to fill a car with one year’s worth of gasoline.

Assuming a distance of 12,000 miles per year and $2.81 per gallon of gasoline, the Ford Fusion will cost $1,349.28 per year to fill with gasoline. However, the Ford Fusion Hybrid will only cost $864.92 per year. Of course this is just a rough estimate, as the price of gasoline often fluctuates.

Next, divide the extra money you would spend buying the hybrid by the extra money you would spend on gasoline for the non-hybrid. This determines how many years it will take for the hybrid to pay for itself. Keeping with the Ford example, a simple crunching of numbers reveals that it will take roughly 17 years of driving a Fusion Hybrid before the $8,255 premium paid over the regular Fusion is worth it.

Unless you plan on driving your Fusion Hybrid for that long, buying one isn’t a good financial decision. Then again, you may have bigger, nobler reasons for investing in one -- like saving the environment.