How to Read Tire Size

December 5th, 2019 by

Profile of black Mercedes-Benz SUV parked on city street

What do tire sizes mean? In short, they ensure you pick the right tire for your vehicle. Next time your Mercedes-Benz is in need of new tires, learn how to tell the tire size by examining the tire markings with Mercedes-Benz of Henderson!

How to Find the Tire Size

Tires are coded with numbers and letters to represent different aspects of the tire size and construction. If you’re wondering how to tell the tire size, simply use this guide:

Tire Type

The first letter indicates the class of the tire. A “P” shows that this is a passenger vehicle tire, an “LT” means the tire is designated for light trucks, and no letters mean that this is a Euro metric tire.

Tire Width

Following the tire class is a three-digit number. This number is the width of the tire in millimeters as measured from one sidewall to another. For example, if you see “P245/”, that means you have a passenger vehicle with a tire width of 245 millimeters.

Aspect Ratio

A two-digit number following the tire width and slash mark, this number is the ratio of the height of the tire’s cross-section to its width.


After the aspect ratio will be a letter indicating the construction of the tire. The most common tire is a radial tire which is noted by the letter “R”. Radial tires are built with layers of fabric with cords that run at right angles to the circumference of the tire. This helps strengthen the tire tread for improved performance.

Wheel Diameter

The next two-digit number is that of the wheel diameter. This specifies the measurement in inches of the wheel a tire will fit. For example, an “18” means your tire will fit an 18-inch wheel.

Load Index

Found after the wheel diameter, this number represents the maximum load the tire can carry while properly inflated. It’s important to have a tire load index that meets or exceeds the specifications set by your vehicle’s manufacturer.

Speed Rating

After the load index number, you’ll notice another letter. This letter represents the speed rating of the vehicle. Typical speed ratings start with “Q” rated tires with a top speed of 99 mph and end with “(Y)” which can handle over 186 mph. It’s important to note, though, that no matter what speed your tires are rated for, you’ll want to follow posted speed limits and consider road conditions while driving.

Additional Tire Markings

While looking at your tire, you may notice some additional markings that aren’t directly related to tire size. These can include:

DOT Symbol

“DOT” stands for the Department of Transportation. This is a required marking to show that the tire has passed all the minimum DOT standards to be sold in the U.S. Following this, there will be a letter and number to show what plant the tire was manufactured at, as well as another two letters noting the size.

Tire Identification Number

If you see a series of letters and numbers following the “DOT” symbol, they are optional markings included by the manufacturer to note things like tire tread, construction, and the week and year the tire was produced.


Uniform Tire Quality Grading, or UTQG, is a system developed by the U.S. Department of Transportation to rate tires based on tread wear, traction, and temperature resistance. Ratings run from A to C for traction and AA to C for temperature, with C being the low end of the scale. Keep in mind these rating tests are performed by the manufacturer and rate durability, not the projected lifespan of the tire.

Maximum Tire Pressure & Load

A number followed by “PSI” is the maximum amount of tire pressure the tire can hold and operate with. A number followed by “LBS” is the maximum load in pounds that a tire can carry when the tires are at maximum inflation. Neither of the maximums are recommended for optimal vehicle performance. Instead, you’ll want to consult your owner’s manual for ideal ranges of tire pressure and load capacity.

What Size Tires Do I Need?

Now that you know how to read tire size, how do you know what tire size your vehicle needs? Your vehicle’s owner’s manual will contain that information, or you can visit Mercedes-Benz of Henderson’s tire center where our expert techs will match your vehicle with the correct tire size.

Find Mercedes-Benz Tires at Mercedes-Benz of Henderson!

When you need to shop for tires for your Mercedes-Benz, Mercedes-Benz of Henderson provides both genuine manufacturer-approved tires and great pricing with our parts and service specials. Reach out to us today, and we’ll find a tire that fits your vehicle, budget, and performance needs!

Posted in FAQ, Service, Service Tips