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How to Read Tire Size

What do tire sizes mean? What size tires do I need? If your Mercedes-Benz is due for new tires, you might be asking yourself these questions. The Mercedes-Benz of Honolulu tire center can help you learn the answers with our guide on how to read tire sizes. 

What Do Tire Sizes Mean?

Tire SizeTire sizes help you pick the right tire for your vehicle. These sizes indicate whether the tire will fit your wheel, as well as whether it will support the needs of your Mercedes-Benz. If you need help choosing a tire for your Mercedes-Benz, reach out to Mercedes-Benz of Honolulu!

How to Tell Tire Sizes: Deciphering the Markings

While tire markings can seem confusing at first, this string of letters and numbers actually represents different aspects of your tire construction and size. Look at the sidewall of your tire, and then compare what you see to our breakdown below:

Tire Type

The first letter you come across signifies the class of the tire. A passenger vehicle is indicated by a “P” while light trucks are indicated by an “LT.” If these letters are absent, that indicates a Euro metric tire. 

Tire Width

The tire class is followed by a three-digit number which indicates the tire’s width. This width measurement is taken in millimeters and is the distance from one sidewall to another. For example, if your tire reads “P345/”, that means you have a passenger vehicle with a tire width of 345 millimeters. 

Aspect Ratio

Following the tire width and slash mark is a two-digit number known as the aspect ratio. This ratio is that of the height of the tire’s cross-section to its width. 


After the aspect ratio is a letter denoting the construction of the tire. Radial tires are noted by the letter “R” and are the most common type of tire. Built using layers of fabric, Radial tires have cords that run at right angles to the circumference of the tire. This method of construction allows for increased strength and performance, as it helps reinforce the tread of the tire. 

Wheel Diameter

Wheel diameter dictates the measurement in inches of the wheel a tire will fit. In other words, the number “20” means your tire will fit a 20-inch wheel. 

Load Index

Load index is the maximum load this tire can carry while properly inflated. This number is found after the wheel diameter. When it comes time to replace your tires, you’ll need to make sure the tire you pick has a load index that meets or exceeds the specifications set by your vehicle’s manufacturer.

Speed Rating

The speed rating of the vehicle comes next and is represented by a letter. Speed ratings normally start with “Q” and offer a top speed of 99 mph. Ratings continue up to “(Y)” which is for tires built for speeds above 186 mph. No matter the actual ratings of tires, you’ll need to keep in mind speed limits and road conditions when navigating Ewa Beach and Waipahu. 

Other Tire Codes & Markings

Once you get past tire size and construction, you’ll see there are more tire markings on the sidewall. Some of these symbols include: 

DOT Symbol

“DOT” stands for the Department of Transportation. This required marking shows that the tire has passed all the minimum DOT standards to be sold in the U.S. On the tails of this marking, you’ll find another letter and number which note the plant the tire was manufactured at. There are also two more letters indicating the tire size. 

Tire Identification Number

Succeeding the required “DOT” symbol, you may find an optional marking consisting of a series of letters and numbers. These can be added by the manufacturer to note things like tire tread, construction, and the week and year the tire was produced. 


Developed by the U.S. Department of Transportation, Uniform Tire Quality Grading, or UTQG, is a system that rates tires based on tread wear, traction, and temperature resistance. Ratings for this system run from A to C for traction and AA to C for temperature (C being the lowest rank.) These tests are run by the tire manufacturers and ultimately focus on durability. They don’t dictate the lifespan expected for the tire. 

Maximum Tire Pressure & Load

If you spot a number followed by the letters “PSI,” that number is the maximum amount of tire pressure the tire can hold and operate with. The maximum load in pounds that a tire can carry when the tires are at maximum inflation is indicated by a number by “LBS.” 

Note: These numbers are not the pressure or load that’s recommended for optimal vehicle performance but rather the absolute maximums at which your tire and vehicle can function. For ideal tire pressure and load capacity numbers, your owner’s manual is a solid reference. 

What Size Tires Do I Need?

So, now you can read tires sizes, but that still doesn’t answer the question, “What tire size do I need?” Your owner’s manual will contain that information, but the experienced technicians at Mercedes-Benz of Honolulu can also take a look and pair you with an appropriately sized tire. 

Find Mercedes-Benz Tire Service and Replacements in Honolulu!

From maintenance and repairs to tire replacements, Mercedes-Benz of Honolulu is your source for Mercedes-Benz tires and care. Schedule a tire consultation today if you need assistance when it comes to how to tell tire size or if you need to purchase a replacement tire!

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