How to Read Tire Sizes

comparing tire wear on two tires

On every tire, you will find an odd string of letters, numbers, and slashes. What does it all mean? Become a tire reading pro in no time by taking it one step at a time.

Find The Standard Size You Need

Thankfully, you do not need to know how to decipher the jumble of letters and numbers on your tires themselves if all you want to do is find out what you need for your vehicle. If your purchasing new tires for the first time or are in the market for a spare, you do not need to get your pants dirty crouching down to inspect your tires. If you go to a tire retailer’s website and input your vehicle, year, make, and model, they will likely have the information readily available. Of course, you can also google, “What size tires fit [insert name and year of your vehicle]?”

How To Read The Tire Itself

To go through this process together, let’s start with what may seem like an indecipherable jumble of letters and numbers: P250/45R18 88Y.

  • Type of Tire: Starting with the simplest thing, “P.” “P” means that this tire is made for passenger vehicles. Your other options are “T” or “LT,” meaning temporary (for a spare) and light truck. But what if you do not see any letter? If this is the case, you are dealing with a Euro-metric tire—which means when you get to the load index section below, and you will need to be looking at a load index chart made according to European standards.
  • Tire Width: Starting with the first three numbers in our jumble, “250,” we find the tire width in millimeters. This is the width of the part of the tire that makes contact with the road. If you are looking for bigger tires than what you have, whether because of upcoming inclement weather or because you’ve got a need for (safe and non-slippery) speed, then you will want to find a bigger set of first numbers.
  • Aspect Ratio: The next group of numbers, “45,” is misleading because it is not just a number, but a percent. There’s a little math involved here: this percentage is found by dividing the height of the tire tread by the width of the tire tread. With aspect ratios, a lower percentage is generally preferable. A lower percentage means that the tire is less thick compared to its width and that usually gives you a more controlled ride.
  • Tire Construction: The “R” in our tire jumble stands for radial, which means that it the strips of rubber comprising the tire run at a 90-degree angle to the tread centerline, like a radius. Essentially every tire is made this way, so it is a bit of a holdover from earlier times. If you see a “B” instead of an R, that means that the tire is bias-ply, and the strips of rubber run at a 30-45 degree angle to the tread centerline instead.
  • Wheel Diameter: Our next group of numbers, “18,” is the diameter of your rim (the wheel that your tire sits on) in inches. Yes, I know it is crazy that we have both the imperial and metric systems in one place! But alas, that’s how it goes.
  • Load Index: We have “88,” which tells us that each tire can withstand 88 weight. But 88 what? It is not as easy as the number of pounds. The easiest way to find out is to look at a load index chart. In our case, 88 means that each tire can withstand 1235 pounds of weight. The amount of load index you will need is typically listed inside your car’s door jam.
  • Speed Rating: Last but not least, we have our “Y” to reckon with. If your tire has a “Y,” that means it is safe to travel at speeds of up to 186 MPH—hopefully, faster than you will ever have an opportunity to drive! From “Y,” only goes down from there: A tire marked “Z” is safe up to 149 MPH, and a tire marked “S” (it should stand for slow, in my opinion) can only safely travel at speeds up to 112 MPH.

How Does My Insurance Policy Cover Tires?

Your coverage will depend on your policy, but frequently, you will have to replace your tires yourself (if they are damaged from normal wear and tear or if you want a spare.

Bottom Line

You do not need to learn to decipher the letters and numbers unless you want to since you can google the info. But if you are going to impress your friends or see if a particular tire will work for your vehicle, it is a good skill to have.

Article Sources

  1. Tire Industry Association. "Reading a Tire Sidewall," Accessed Oct. 2, 2019.

  2. Goodyear Tires. "Tire Speed Rating," Accessed Oct. 2, 2019.