Tire Size Explained: What the Numbers Mean | Les Schwab (2024)

Whether you’re switching out your tires for a different look or added performance, a good place to start your search for that new tread is the sidewall on one of your existing tires. The numbers and letters on that sidewall represent everything from the tire type and width to the aspect ratio, construction type, diameter, load index, and speed rating. Here’s how to read the sidewall on your tires.

Tire Size Meanings

The numbers and letters on the sidewall can be confusing. Here’s what each one means.

A: TIRE TYPE

The first letter in the code tells you what class of tire it is.

If there’s no letter before the first number, you have a metric tire most commonly referred to as European size. It’s also measured in millimeters but may have a different load capacity than a P or LT tire. Confused by all the numbers and letters? The pros at Les Schwab can help.

B: TIRE WIDTH

The three-digit number following the letter is the tire’s width (from side to side, looking at the tire head-on) in millimeters. This may also be referred to as the section width.

C: ASPECT RATIO

The forward slash separates the tire width number from the two-digit aspect ratio. The bigger the aspect ratio, the higher/taller the tire’s sidewall, or “profile” as it’s sometimes called.

The aspect ratio is indicated on the tire sidewall as a percentage. It’s the height of the sidewall measured from wheel rim to top of the tread, expressed as a percentage of tire width.

In this example, the aspect ratio is 65, meaning the sidewall is 65 percent as high as the tire is wide. To get the sidewall height, take the tire width of 215 mm and convert it to inches (8.46). Then multiply this by 65% (.65). This gives you an answer of 5.5, the sidewall height in inches.

D: CONSTRUCTION TYPE

This single letter tells you about the internal construction of the tire. These include radial and bias.

E: WHEEL DIAMETER

This two-digit number specifies wheel diameter in inches. It’s the distance between the two bead seat areas (where a tire gets tightly sealed onto the wheel).

F: LOAD INDEX

The two-digit or three-digit number that follows the gap specifies tire load index.

The load index symbol indicates how much weight a tire can support, based on the following standard chart (see below). In our example, the load index is 89. This means the tire has a load capacity of 1,279 pounds, when inflated to the tire’s maximum air pressure rating.

Please note: Some run-flat tires are identified with an F followed by the type of internal construction.

G: SPEED RATING

The last letter is the tire speed rating.

This indicates the top speed it’s safe to travel for a sustained amount of time. A tire with a higher speed rating can handle heat better and provide more control at faster speeds. The maximum operating speed of a vehicle is no more than the lowest speed rating of all tires mounted on the vehicle. (Of course, you should always abide by speed limits for safer driving.) Speed rating is usually, but not always, a single letter (see the chart below).

Tire Size Charts

Below you will find several charts that will help you understand tire sizing numbers, including a load index chart and speed rating chart.

Buying New Wheels or Changing Your Tire Size?

A tire size calculator is a quick way to ensure a new set of tires will fit your car, SUV, sports car, light truck or crossover. But remember, this calculator is only an estimate. It’s important to stay within the sizing tolerances of your vehicle. Tires that are the wrong size could cause some pull in the steering wheel, rub against the suspension or body of your vehicle, reduce clearance on hills, or result in a stiffer or noisier ride.

If you’re considering mounting a different tire size on your vehicle, stop by Les Schwab. We’ll help you make the right choice about the tires and wheels for your vehicle’s suspension, gearing, and bodywork. See how new tires and rims will look on your car or truck. Start shopping now.