Do You Know What All the Letters on Your Sidewall Mean?

Tire-roglphyics

Harry WagnerPhotographer, Writer

Tires are your vehicle’s connection to the road, and they are one of the biggest expenses associated with your vehicle. As a result, tire shopping can be intimidating. Metric and standard sizing, LT versus P Metric, load range and rating—it can be overwhelming, but it doesn’t have to be. We are here to help you decipher the information on your tire’s sidewall and help you make sense of it all.

In general, the load range (expressed as C, D, E, and so on) corresponds to the ply rating. C is a six-ply rating, D an eight-ply, E a 10-ply. This is not the actual number of plies but rather a rating system that dates back to when bias-ply tires were common. Radial construction is the most common in modern tires, but bias-ply tires are still offered for off-road use. A radial tire allows the sidewall and the tread to function as two independent features of the tire. As the name implies, radial tires use steel belts that travel around the radius of the tire carcass at a 90-degree angle to the center of the tire. As a result, the ride quality and tread life tend to be better on a radial tire than a bias-ply tire.

The DOT Tire Identification Number is perhaps the most mystifying piece of information on your tire’s sidewall. DOT stands for Department of Transportation—that part’s easy enough. This is followed by 10 to 12 characters that can be used to identify the manufacturing location, tire size, and manufacturer's specifications, along with the week and year the tire was manufactured. For instance, 73BNP4F2814 breaks down as follows: 73 is the plant code, BN is the tire size, and P4F is the tire type. This information is only really useful to us in the event there is a tire recall. The last four numbers are important though, particularly when buying used tires. The month and year this tire was manufactured was “2814”—the 28th week of 2014.

Another consideration is how a larger tire will affect your speedometer. We often here “my speedometer is 5 mph off” but your speedometer is actually a percentage off, not a fixed number of miles per hour. You can calculate the percentage by dividing the new tire size by the old tire size. And not just the overall diameter. For the most accurate measurement, check the static load radius of the tire, or the distance from the ground to the center of the hub. Take this measurement and double it to get your actual working diameter. Note that all tires tend to measure smaller than they are labeled, and the amount varies by model and manufacturer, so to be accurate you need to physically measure the height of the old tire and the new tire. More math formulas can be found in the captions; they are key to deciphering the information on your sidewalls.