To paraphrase Mark Twain, it's not what you don't know that can come back to bite you; it's what you know for sure that ain't true. When it comes to maintaining your car, misconceptions abound. And even the best intentions can lead you to spend more money than necessary or even compromise your safety. Here are common myths that can do more harm than good:

Myth: Engine oil should be changed every 3,000 miles.
Reality: Despite what oil companies and quick-lube shops often claim, it's usually not necessary. Stick to the service intervals in your car's owner's manual. Under normal driving conditions, most vehicles are designed to go 7,500 miles or more between oil changes. Changing oil more often doesn't hurt the engine, but it can cost you a lot of extra money. Automakers often recommend 3,000-mile intervals for severe driving conditions, such as constant stop-and-go driving, frequent trailer-towing, mountainous terrain, or dusty conditions.

Myth: Inflate tires to the pressure shown on the tire's sidewall.
Reality: The pounds-per-square-inch figure on the side of the tire is the maximum pressure that the tire can safely hold, not the automaker's recommended pressure, which provides the best balance of braking, handling, gas mileage, and ride comfort. That figure is usually found on a doorjamb sticker, in the glove box, or on the fuel-filler door. Perform a monthly pressure check when tires are cold or after the car has been parked for a few hours.

Myth: If the brake fluid is low, topping it off will fix the problem.
Reality:
As brake pads wear, the level in the brake-fluid reservoir drops a bit. That helps you monitor brake wear. If the fluid level drops to or below the Low mark on the reservoir, then either your brakes are worn out or fluid is leaking. Either way, get the brake system serviced immediately. You should also get a routine brake inspection when you rotate the tires, about every 6,000 to 7,000 miles.

Myth: If regular-grade fuel is good, premium must be better.
Reality:
Most vehicles run just fine on regular-grade (87 octane) fuel. Using premium in these cars won't hurt, but it won't improve performance, either. A higher-octane number simply means that the fuel is less prone to pre-ignition problems, so it's often specified for hotter running, high-compression engines. So if your car is designed for 87-octane fuel, don't waste money on premium.

Myth: Flush the coolant with every oil change.
Reality:
Radiator coolant doesn't need to be replaced very often. Most owner's manuals recommend changing the coolant every five years or 60,000 miles. Of course, if the level in the coolant reservoir is chronically low, check for a leak and get service as soon as possible.

Myth: After a jump-start, your car will soon recharge the battery.
Reality:
It could take hours of driving to restore a battery's full charge, especially in the winter. That's because power accessories, such as heated seats, draw so much electricity that in some cars the alternator has little left over to recharge a run-down battery. A"load test" at a service station can determine whether the battery can still hold a charge. If so, some hours on a battery charger might be needed to revive the battery to its full potential.

Myth: Let your engine warm up for several minutes before driving.
Reality:
That might have been good advice for yesteryear's cars but is less so today. Modern engines warm up more quickly when they're driven. And the sooner they warm up, the sooner they reach maximum efficiency and deliver the best fuel economy and performance. But don't rev the engine high over the first few miles while it's warming up.

Myth: A dealership must perform regular maintenance to keep your car's factory warranty valid.
Reality:
As long as the maintenance items specified in the vehicle owner's manual are performed on schedule, the work can be done at any auto-repair shop. If you're knowledgeable, you can even do the work yourself. Just keep accurate records and receipts to back you up in case of a warranty dispute on a future repair.

Myth: Dishwashing and laundry detergents make a good car wash.
Reality:
Detergent can strip off a car's wax finish. Instead, use a car-wash liquid, which is formulated to clean without removing wax.