How to Read Tire Size

January 16th, 2020 by

Service advisor and customer examining wheel on blue Porsche on lift

Considering a new set of tires for your Porsche and wondering, “What do tire sizes mean?” The tire size indicates the size of the tire and whether it will fit your vehicle. When you’re due to replace your Porsche N-Specific tires, find out how to tell the tire size by examining the tire markings with Porsche Fremont!

How to Find the Tire Size

Take one quick glance at any tire, and you’ll see numbers and letters on the sidewall. These act as codes and signify different aspects of the tire size and construction. To better understand how to tell the tire size, use the following guide:

Tire Type

The class of the tire is indicated in the first letter. A passenger vehicle tire is represented with a “P,” an “LT” indicates the tire is designated for light trucks, and the absence of letters means that this is a Euro metric tire.

Tire Width

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

Aspect Ratio

The aspect ratio is right behind the tire width and slash mark. This two-digit number is the ratio of the height of the tire’s cross-section to its width.


Trailing the aspect ratio will be a letter indicating the tire’s construction. A radial tire is the most common tire and is noted by the letter “R.” Layers of fabric with cords are used to build these tires, and they run at right angles to the circumference of the tire. This type of tire construction helps improve performance by strengthening the tread of the tire.

Wheel Diameter

The wheel diameter is the next two-digit number. This specifies the measurement in inches of the wheel a tire will fit, i.e., a “17” means your tire will fit a 17-inch wheel.

Load Index

Representing the maximum load the tire can carry while properly inflated, this code is found after the wheel diameter. Note: When selecting a tire, it’s important to have a tire load index that meets or exceeds the specifications set by your vehicle’s manufacturer.

Speed Rating

Speed ratings are indicated by a letter after the load index. Speed ratings typically start with “Q” tires, which are rated for a top speed of 99 mph, and end with high-performing “(Y)” tires, which can handle over 186 mph. Note: No matter your tire’s speed rating, you’ll want to make sure to take into account road conditions and posted speed limits while driving around the Bay Area.

Other Tire Markings

While we’ve covered how to tell tire size, you may notice other coding on your tires that you’re curious about. The following markings that aren’t directly related to tire size but can be helpful to know:

DOT Symbol

“DOT” is a required marking and stands for the Department of Transportation. This is present to show that the tire has passed all the minimum DOT standards to be sold in the United States. Following this, you may find a letter and number to show what plant the tire was manufactured at, in addition to two more letters noting the size.

Tire Identification Number

Following the “DOT” symbol, there’s sometimes a series of letters and numbers. These are optional manufacturer markings to note things like tire tread, construction, and the week and year the tire was produced.


UTQG stands for “Uniform Tire Quality Grading” and is a system developed by the U.S. Department of Transportation to rate tires based on temperature resistance, tread wear, and traction. Ratings run from AA to C for temperature or A to C for traction, with C being the bottom of the scale. Manufacturers perform these tests to rate durability, but they’re not indicative of the projected lifespan of the tire.

Maximum Tire Pressure & Load

If you spot “PSI” right after a number, this number 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. Note: It’s not recommended that you hit these maximums. For optimal performance, you’ll want to consult your owner’s manual for ideal ranges of tire pressure and load capacity.

What Size Tires Do I Need?

So, you know how to tell tire size, but how do you know what tire size your vehicle needs? One easy way to find out is to check your vehicle’s owner’s manual, as it should contain that information. Or, if you’re visiting Porsche Fremont’s tire center for service, our knowledgeable techs will help match your vehicle with the correct tire size.

Get Porsche N-Specific Tires at Porsche Fremont!

When shopping for Porsche N-Specific tires, you’ll find a great selection and helpful technicians at Porsche Fremont near San Jose and Palo Alto. From matching you to new tires to helping you maintain your tires through balancing and alignments, we’re dedicated to keeping your Porsche performing at its best!

Posted in FAQ, Service, Service Tips