Understanding Tire Size
Aug 13th 2018
Maybe you know your tire size is a series of numbers and letters printed somewhere on the sidewall, but maybe you’re not sure where. Or even which series of numbers and letters it is. There’s a lot of stuff on there, after all, and we’ve even talked about some of it before in our post about speed and load ratings. And, while the size can also be found on the door of your vehicle, we usually recommend getting it from your current tires. Some vehicles have alternate sizes or have had their size changed by a previous owner, so it’s best to know what’s actually on there. So how can you find it? And what exactly do those numbers and letters mean?
Well, let’s take a look...
The first part of your size is usually going to be a letter. This letter indicates what type of vehicle or use the tire is intended for. Commonly, you’ll find a P there, meaning the tire is meant for “Passenger Vehicles,” but you might also see LT for “Light Truck” or ST for “Special Trailer.” Basically, this letter means that the tire meets the basic standards of use in the US for a certain type of vehicle. Sometimes, there may not even be a letter at all. (This just means the tires are Euro-Metric and might have a different load index. You can still use them.)
The first 3-digit number represents the tire width. The width is measured from sidewall to sidewall in millimeters, and that measurement is exactly what the first number is. So, in the example above, the first number is 225 and this means the tire is 225 millimeters wide from sidewall to sidewall.
The 2-digit number you encounter after the slash isn’t as straightforward as the first one. This number is the aspect ratio. Instead of being a 1-1 measurement, this number represents the ratio of the height of the tire's cross-section to its width. In the size above, for example, the number is 60, this means the height is equal to 60% of the tire’s width. The bigger that number is, the taller the sidewall will be.
Here you’ll find the other letter you’ll encounter in tire size, and this time it represents the tire’s internal construction. The most common, and therefore what you’ll usually see, is the letter R which stands for a Radial construction. This basically means the plies are arranged radially. The only other one you might come across is D, which indicates Diagonal (or bias) construction. It doesn’t affect the tire dimensions.
The last part of your size is the bit you’re most likely to already know: the diameter. You may have also heard this number referred to as the rim size, that’s because that’s basically what it tells you: the size of the wheel it’s meant to fit on. This is measured in inches from one edge to the other. In the example above, the diameter is 15 meaning the tire is intended for a 15” wheel.
That wasn’t so bad, right? It looks more complicated than it is and to be honest, you don’t really need to know all the details to buy a tire. However, some understanding never hurts! This way you can at least be sure you’re getting the right type of tire as well as size.