What do those numbers on your tires actually mean?
After struggling to settle on a design, the Army Corps of Engineers finally builds New Orleans some new levees, only to realize that there are a few holes. Image courtesy of awm.delaware.gov
Tires come in a variety of sizes, and each number on your tires corresponds to a certain dimension. These descriptions will demystify the several types of tire sizes, making it a bit easier for you the next time you need to replace your tires. The groups of numbers mentioned refer to the tire code, marked on every tire, and they occur after a letter; “P” for passenger cars, “LT” for light trucks, “ST” for trailers, and “T” for temporary tires, such as spare tires.
Tire width: This is the first group of two or three numbers on the tire. The wider the tire, the more surface area you have contacting the road. While this will allow you to handle better on dry pavement, your gas mileage will decrease as a consequence. In fact, if your tires are too wide, they’ll chafe against the car body and can wear unevenly, or even blow out entirely. If you’re going to drive in wet weather, you will want more narrow tires. Remember, though, that less surface area will make it more difficult to drive on in drier climates, and too narrow a tire can be dangerous in itself. Generally, you should go with the width your car manufacturer suggests, unless you’re receiving some professional tire advice.
Aspect ratio: This number, which is two to three digits, and occurs after the width, tells you about the size of the tire’s sidewall. Lower sidewalls, like those found on sports car tires, will give you superior cornering, but the ride can be much harsher with less space between you and the road. Also, if you change your sidewall too radically from the manufacturer’s suggestion, your speedometer can give you inaccurate readings. If you wish to make a big change to your sidewall size, talk to your mechanic about changing wheel sizes to compensate and ensure an accurate speedometer readout.
Wheel diameter: The last group of two numbers. You’ll need tires that fit your wheel diameter properly, and as mentioned above, if you’re changing wheel size, you’ll need to make sure to find a tire that fits properly.
—Seth Berger
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