SOUTH BEND, Ind. (CNNMoney) -- Most people probably don't realize how easily a brand new set of tires can turn the car they love into something awful.
"They can transform your vehicle," said Jennifer Stockburger, a tire tester for Consumer Reports magazine.
I knew that, too, at least in a theoretical sense. But I learned first-hand when I got to drive four otherwise identical BMW 3-series sedans each with a different set of performance tires.
I can now tell you that the worst BMW I've ever driven was a brand-new 3-series on cheap tires.
I was visiting the South Bend, Ind. headquarters of the tire sales Web site, TireRack.com. Even before I left the Tire Rack parking lot on the first set of tires I knew something was badly wrong. The BMW felt like it was driving over a gravel road... even when I was driving over smooth asphalt . The steering also felt indistinct and vague, not the way BMWs usually feel.
Later on, I drove the car with a set of more expensive, and evidently better, tires and suddenly the BMW felt like a BMW. The ride was smooth and the steering responsive with good, but not irritating, feedback.
Then came the fun part: testing each set of tires on a water-soaked test track.
Sure enough, the same tires that felt best on the road also excelled on the wet track.
The cheap tires felt fine going around a tight turn until, with no warning, the back end of the car swung out like a pendulum. Even with the help of electronic stability control -- which was set to "Sport" mode to allow some sliding -- it was hard to get the car back in line. (ESC pumps the cars brakes at individual wheels to help get the car back in line when computers sense a skid.)
On the more expensive tires, I was able to make a more graceful trip around the wet track. When the tires did start to skid, they did so gradually and, when ESC intervened, it did so gently and predictably.
How to shop: That's good to know, but most of us don't get to test-drive tires. The secret is knowing where to find good information.
Start by knowing what's on your car now. Your car's suspension, steering and braking systems were designed around a specific tire and your car will usually perform best with tires that most closely match the originals, said Stockburger.
If you don't feel like parsing all the numbers on the side of your tire -- although it might still be good to do that -- the Tire Rack's Web site allows you to enter the make, model and trim level of your car and see a list of tires that will work.
That doesn't mean you can't change things up a little to suit your needs or your tastes, said John Rastetter, Director of Tire Information Services at Tire Rack.
For instance, if your car came with "V" rated tires, meaning they're designed to handle speeds up to 149 miles per hour, you might safely replace them with "H" rated tires designed to go only 130 mph, he said. You might get a lower price and a more comfortable ride, just be sure you're not going to drive aggressively.
While consumer automotive consultant Lauren Fix said she agrees with Rastetter, not everyone is comfortable with that idea.
Consumer Reports, for instance, recommends against buying a lower speed-rated tire, said Stockburger, except when buying a dedicated winter-only tire. A lower speed-rated tire can also have worse traction and braking performance, she said, even though that's not, technically, what those ratings are about.
Many car owners are now paying attention to rolling resistance, too. Low rolling resistance improves fuel economy.
But the actual difference in fuel mileage is minimal -- about 1%, at best -- and the cost, besides a higher-priced tire, is often longer stopping distance, said Stockburger. In an emergency, you could find that you made a regrettable trade-off.
A decision guide in the Tire Rack Web site can steer you toward tires that fit your needs and preferences while still staying within reasonable parameters for your car.
Both Tire Rack and Consumer Reports' Web sites offer detailed breakdowns of various tires' performance characteristics and the Tire Rack, in particular, has owner ratings that tell you what people who've lived with the tires actually thought of them.
One thing you should definitely not do, though, is wait until the last minute to start shopping for new tires.
"There's a huge advantage to shopping around and comparing prices," said Stockburger.
With some time and attention, you can find the right tires at the best price. 
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