We are always confusing about our tires. Some numbers on the sidewall are listed in metric while others are in inches.
Here’s the information on what all the sidewall numbers and letters mean.
Tire Size Meanings
A: TIRE TYPE
The first letter in the code tells you what class of tire it is.
P stands for passenger vehicle tire. P-class tires include cars, SUVs, crossovers, minivans and smaller pickup trucks.
LT means light truck tire, designed for vehicles that are towing trailers or have ¾- and 1-ton load capacity.
ST stands for Special Trailer. These tire sizes are meant for trailers, including fifth wheels and other travel trailers, boat trailers, and utility trailers.
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.
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. Also called the section width, this measurement is taken from outer sidewall to inner sidewall.
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 a percentage. It’s the height of the sidewall measured from the wheel rim to the top of the tread, expressed as a percentage of tire width. In other words, it’s sidewall height divided by tire width.
In this example, the aspect ratio is 75, meaning the sidewall is 75 percent as high as the tire is wide.
D: CONSTRUCTION TYPE
This single letter tells you about the internal construction of the tire. R is for radial tires, D is for tires built with diagonal (crisscrossed) plies, called bias-constructed tires.
E: WHEEL DIAMETER
This two-digit number specifies wheel diameter in inches, how wide the wheel is across the center. It’s the distance between the two bead seat areas (where a tire gets slotted and tightly sealed onto the wheel).
F: LOAD INDEX
The two-digit or three-digit number that follows the gap specifies the tire load index. The load index symbol indicates how much weight a tire can support, based on the following standard chart. In our example, the load index is 89, which indicates the tire has a load capacity of 1,279 pounds.
G: SPEED RATING
The last letter is the speed rating, which tells you the top speed it’s safe to travel at 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.) The speed rating is usually, but not always, a single letter (see the chart).
