Victoria Car & Vehicle Maintenance - Tires & Wheels - Getting the Right Size
Once you've identified what type of tire you want, be sure to get the correct size for your vehicle. A single make and model of tire can come in several different combinations of width, height and diameter.
To find out what size your vehicle requires, check the sidewall of the tires you've got now. The size should also be printed on the driver's door, in the owner's manual or glove box. What you'll find is a combination of letters and numbers, such as P245/35R19. "P" indicates the tire is meant for a passenger vehicle (car, minivan, etc.), a "LT" means light truck or SUV, and "T" for truck. Light-truck tires are designed to have higher-load carrying capacities and are usually found on pickups and SUVs. The number following it, in this case "245", is the tire width in millimetres and "55" is the height expressed as a ratio of the width. "R" indicates a radial tire (as opposed to cross-ply, a style that fell out of favor in the 1970s and "19" is your vehicle's wheel (rim) diameter, in inches. High performance tires usually have a lower aspect ratio than other tires as they have the ability to hug the road when taking turns at higher speeds. This may sound complicated, but as long as you match the letters and numbers on your existing tires, you'll do fine.
Using a shorter or taller tire will compromise the handling and fuel economy of your vehicle, as well as throw off your speedometer, odometer readings and onboard computer. Your mechanic, however, may suggest a slight variation in night or width of the same diameter to improve handling, which you should consider. You might watch those car makeover shows (like Pimp My Ride, and Overhaulin') and watch the show replace the tires and rims for a sportier look with more chrome and less rubber. The tire they choose for that new larger rim (which has a larger last number) will be offset with a smaller second number, so the tire's radius and diameter should remain the same.
One some high performance tires this code is followed by a 4 character code like "102T", which designates the load index and the speed rating for the tire. Different numbers correspond to different maximum loads in lbs. (pounds) and in kg (kilograms),which must be supported by the tire pressures. The speed rating is the maximum safe service speed of a passenger car tire. The speed rating translates into the tire's ability to dissipate heat, or prevent heat build-up, which is a key cause of rubber breakdown and wear. Here are the rating indicators and their mile-per-hour equivalents (this does not yet apply to light truck tires): Q 99 mph, S 112 mph, T 118 mph, U 124 mph, H 130 mph, V 149 mph, W 168 mph, Y 186 mph, Z Above 149 mph. For typical consumers doing highway driving at the speed limit, a Q or S speed rating would be acceptable.
Also on the side of your tires you will also find tread wear, traction, and temperature ratings. Tread wear ratings come from government run tests. The higher the rating the longer the tire can be expected to last. Traction is tested by measuring a tire's ability to stop on paved, wet asphalt and wet concrete. The ratings range from "AA" (best) to "A," "B," and "C." Temperature ratings are either "A," "B," or "C," which are based on correctly inflated tires with good tread, based on a safe speed and a modest load. These will give you a a indication of the tire's quality.