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Size Matters When Choosing Tires

By Cheryl Jensen

Tires are extremely important to your vehicle's performance and safety. After all, it is only those four patches of rubber that connect you to the road. 

When you check tire pressure, which of course you do every week, make sure there is enough tread on the tire to operate safely and that the tires are wearing normally. All grooves should be visible and you can check the tread using a Lincoln-head penny. Insert the penny, head first, into the most worn part of the tire. If the tread is not deep enough to at least touch the top of Lincoln's head, it's time to replace your tires. If you see the treadwear warning bars across the tire, it's way past time to replace that tire.

If this is the first time you've ever replaced those tires, the most important place to start is to replace your current tires with tires of the type and same size. Having the right size tire is the most critical element affecting the ride comfort and safe handling of your car.

"Tires that are too large could cause serious handling and steering problems," said John Buettner, director of Technical Services, Tire Association of North America.

Where do you find that information? Start by studying the writing on the wall, the sidewall of your current tires, that is. Embossed on the side of every tire is important information regarding the tire and its size. It's contained in a series of nine numbers and letters, such as P215/65R15. "P" means a passenger-car tire. Light-truck tires have the letters LT, instead of P, preceding the size designation. However, most light pickups, sport-utility vehicles and vans use passenger-car tires. The next three digits "215" are the tire's cross-section width in millimeters. Pay special attention to the two numbers listed before the letter "R." These two numbers designate the tire's aspect ratio — the ratio of the height to the width of the tire. For example, a tire with the size designation of P215/65R15 has an aspect ratio of 65, meaning the tire is 65 percent high as it is wide. "R" means a radial tire, which is virtually the only tire design sold today. The next two digits, 15, are the diameter of the wheel on which the tire fits, in inches.

It's important to double check this information, especially if you have bought a used car. Used cars could have the wrong size tires because the previous owner may not have followed the manufacturer's tire recommendations. You can find the information in the owner's manual, the placard which is typically located on the door jamb, glove box or inside of the fuel filler door. Although there are many categories of tires, most likely your vehicle came with "all-season" radial tires. 

It's important to replace all four tires at the same time to maintain the traction balance from front to rear. If you are on a budget and thinking of buying only two tires "consult your tire expert and or vehicle manufacturer for best position for your type of vehicle. This may differ but as a rule they should go on the axle that is driving the vehicle. However, this can vary according to type of vehicle" said Buettner.