Monday, September 30, 2013

Tire Size Chart: Find the Right Size

Reading a tire size chart can be confusing, whether you're actually intending to replace your tires soon, or just casually checking prices. The following information will help you read these charts correctly and identify the proper size for your vehicle. 
To find the correct tire size for your vehicle, first look closely on the sidewall of one of the front tires. On the tire sidewall (the part of the tire that you can actually wash), look for a sequence of letters and numbers in the pattern of the following tire size measurements: 185/60R-14, 225/50R-16 or 255/40ZR-18 for example. Don't worry if your tires don't match one of those 3 size examples; just jot down the exact size listed on the sidewall of your front tire. You should also check one of the rear-tire sidewalls, in case the two rear tires need to be a bit larger than the two front tires. Your owner's manual should help you confirm the size(s) that you have written down. 

For example, let's imagine that you've written down the following tire size directly from the sidewall: 225/50R-16. The first part of this measurement describes the width of the tire in millimeters; in this example, the tire is 225 millimeters, or roughly 8.9 inches, wide (225mm/25.4mm per inch = 8.86 inches). 
Now that the tire width has been identified as 225mm, we should consider the next part of the data in our example: 50R. Specifically, the numerical portion (50) tells us that the tire sidewall height (or aspect ratio) is 50% of the tire's width; in this example, the "height" of the tire is 112.5mm (or 4.43 inches). Finally, the letter R indicates that our example is notdesignated as an ultra-high speed tire; if it were so designated, it would have carried a "ZR" rating (for example, 225/50ZR-16). Only tires rated to cruise at speeds above 150mph can earn the ZR rating. Other speed ratings are designated by other letters.  
The last part of the tire-size rating indicates the wheel-size matchup. From our example, we can quickly determine that the tires will match up with wheels that are 16 inches in diameter. 
As listed on a tire comparison chart, each dimension of the tire (width, height, speed rating and wheel size matchup) is important, because it corresponds to key dimensional and safety data for a particular vehicle. For example, if the tires are too wide or too high, they may rub against the body/chassis of the car while turning or when going over bumps. Moreover, an incorrect tire size can confuse the wheel speed sensors on the car, which could lead to anti-lock braking system (ABS) trouble. Furthermore, if your vehicle has 16-inch wheels, and you've ordered tires for 18-inch wheels by mistake, then you will simply have to send them back.
Thus, it's best to take your time, to identify the exact size of the tires on your vehicle, and to confirm the original manufacturer's recommended tire size (with your owner's manual or a well-informed mechanic), before scanning a tire size chart and placing an order.

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