How to read tire sidewall

Updated on 12.24.2020

Have you ever been shopping for tires? Looked at the tire sizing chart and thought, what the hell am I looking at? After reading this short guide, you'll be a master at reading and understanding tire measurements.

There are two types of size measurements: metric and flotation.

Metric covers a majority of tires people buy and it's more difficult to understand, so we're going to go ahead and start there. There are seven sections for metric sizing. First letters designate

• P – passenger
• LT – light truck
• ST – special trailer
• T – temporary rated tires.

If a tire doesn't specify it, then it's most likely a passenger tire. Just be sure to double-check the tires load rating.

Up next is the section width or the width from the sidewall to sidewall in millimeters.

Following that is the aspect ratio, which we'll explain when we get down to the hard numbers.

Next, you'll see an R which stands for radial.

The next number is a wheel size in inches.

The last thing you'll see is the load rating and speed rating.

So let's look at how these numbers translate to a real tire.

This tire doesn't list P or LT, so it's a passenger tire. The 265 means the width is 265 millimeters or 10.4 inches wide. Then we have the aspect ratio, this number represents the size of the sidewall compared to the section width. The 50, in this case, means that the sidewall is 50% of the width or 5.2 inches.

The R just means radial construction as opposed to bias ply. And the final number, 20, that's the wheel size. Now that we've got the width, sidewall size, and wheel size, we can take all that together and get the height of the tire.

• Width: 10.4"
• Sidewall: 5.2"
• Wheel: 20"

We'll take the sidewall size, add the wheel size, and then add the sidewall size one more time.

Now that (10.4"+5.2"+5.2"+20") gives us a height of 30.4 inches.

Let's take a look at another tire, the LT 275/65R20.

The LT stands for a light truck. The width is 275 millimeters or 10.8 inches. The sidewalls are 65% of 10.8 inches which equals seven inches and it's radial construction. It also fits a 20-inch wheel. The tire's overall height is 34.1 inches.

So one last example of metric sizing, LT 315/75R16. This is a light truck tire that is 315 millimeters wide or 12.4 inches. Sidewalls are 9.3 inches and it fits a 16-inch wheel for a total height of 34.6 inches. If you don't want to do all the math yourself, we've got a handy calculator.

What does ZR mean on tires?

For some reason, tire makers decided to mark high speed rated tires by additional Z letter. We think this made just for a cooler look. If you're driving Type-R, or GT-R or any other GT-Super-High-Speed-Vehicle-R, you're probably going to love that "sporty" touch. But the reality is trivial: ZR stands for tires, that have a speed rating of more than 149mph, so that includes V, W and Y rated tires

Flotation sizing is even easier than the metric.

Here's just how easy it is. A 40 x 15.5 R22LT is 40 inches tall, 15.5 inches wide, and fits a 22-inch wheel.

What is tire speed rating

The Speed Rating - the speed symbol on the sidewall indicates the maximum design speed of the tire.

As with the Load Index, there is a table of speed index values with values ranging from A (minimum value) to Z (maximum value). However, with one exception: the letter H falls out of the sequence and is between U and V, corresponding to speeds up to 210 km/h.

Failure to match the speed index of the tire with the maximum speed of the car (in case the maximum speed of the car is higher than the speed index of the chosen tire) may result in a rapid loss of the tire's performance characteristics, up to bursting of the tire.

Tire speed rating chart

What is tire load index

Tire Load Index is a number that corresponds to the load a tire can carry at its maximum internal air pressure.

Improper matching of the load index to the weight of a loaded vehicle (if the vehicle weight exceeds the value of the tire speed index) may result in rapid loss of tire performance and may even result in the tire exploding.

Tire load index chart