Tires 101

There’s quite a bit of information on the sidewall of every tire, but it’s written in an alpha-numeric code and can be hard to understand. The size of the tire is just one small piece of this code. The tire size P325/60R20 or 35×12.50R20LT may not mean much to the average person, but it can be extremely important when purchasing the correct set of tires for your truck. In Tires 101, we discuss how to decipher the codes on a tire.

How to decipher inch sizes: 

35×12.50R20LT 125Q 12PR

Diameter: 35

The first number is the overall diameter of the tire. The diameter of this tire is 35 inches.

Section Width: 12.50

The second number is the width of the tire in inches. The width of this tire is 12.5 inches.

Tire Construction: R

The tire construction of this tire size is ‘R’ for radial. This letter describes how the tire is constructed.

Wheel Diameter: 20

The wheel diameter of this tire size is 20 inches. The tire is designed to be mounted on a 20 inch wheel.

Tire Class: LT

The tire class of this tire size is ‘LT’ which means it is a Light Truck tire.

Load Index: 125

This number is the load index and the maximum amount of weight the tire can carry. A higher number means the tire can carry more weight. The load index of this tire is ‘125’ – 3,6384 lbs.

Speed Rating: Q

The speed rating of this tire size is ‘Q’, or 99 mph. This letter is the maximum speed rating of the tire.

Load Range: 12

The tire load range of this tire size is 12, which means it has a 12-ply tread. This can be a letter or a number, other load ranges include B: 4-ply, C: 6-ply, D: 8-ply, E:10-ply, and F:12-ply.

See the tables below for Load Index, LT Load Ranges, Speed Ratings, and Tire Construction.

How to decipher metric sizes:

P325/60R20 10PR 126/123R

Tire Class: P

The tire class of this tire is ‘P’ which means it is a passenger car tire. The first letter in the tire size designates the tire class. An ‘LT’ would be a light truck class and no letter would be a euro-metric tire. Note: ‘P’ Tires includes cars, minivans, sport utility vehicles and light-duty pickup trucks (typically 1/4- and 1/2-ton load capacity).

Section Width: 325

The first number in a metric tire size is the section width of the tire. The width of this tire size is 325mm.

Aspect Ratio: 60

The second number in a metric tire size is called the aspect ratio. The aspect ratio on this tire size is 60, which means that the sidewall height is 60% of the section width.

Tire Construction: R

The tire construction of this tire size is ‘R’ for radial. This letter describes how the tire is constructed.

Wheel Diameter: 20

The wheel diameter of this tire size is 20 inches. The tire is designed to be mounted on a 20″ wheel.

Load Range: 10

The tire load range of this tire size is 10 which means it has a 10-ply tread. This can be a letter or a number, other load ranges include B: 4-ply, C: 6-ply, D: 8-ply, E:10-ply, and F:12-ply.

Load Index: 126/123

This number is the load index and the maximum amount of weight the tire can carry. A higher number means the tire can carry more weight. Since LT tires are commonly used on trucks with dual rear wheels, they are branded with two load indexes. The first number indicates the load-carrying capacity if the tire is installed on a truck with a single-wheel rear axle, and the second number applies when the tire is used in a dual rear application. The load index of this tire is “126” – 3,748 lbs for a single-wheel rear axle, and “123”  – 3,417 lbs for a dual rear wheel application.

Speed Rating: R

The speed rating of this tire size is V – 106 mph. This letter is the maximum speed rating of the tire.

Converting Metric to Inches

Now we will convert a metric tire measurement into inches, so it might help to have a calculator handy.

Let’s use the same metric tire from above as an example: 325/60R20

On metric tires, the first number is the Section Width of the tire in millimeters. In this example it reads 325mm. To convert 325mm to inches you simply divide 325 by 25.4, since there are 25.4 mm in one inch. So the width of a 325mm tire is 12.80 inches. The next number is typically referred to as the Aspect Ratio or the proportional relationship between the width and height. This tells you the sidewall height of the tire. For our example, the sidewall height of the tire would be 60% of 12.80″ which is 7.68″. Finally, to calculate the overall diameter of the tire you simply need to add the sidewall measurement twice (for the top and bottom sidewall), and the diameter of the wheel.

If we add the sidewall twice, it is 7.68 + 7.68 = 15.36. Next add 20″ for the wheel diameter, so the tire height 15.36 + 20 = 35.36. According to these calculations, a 325/60R20 metric tire would result in an inch measurement equivalent of a 35.36X12.80R20 tire. As you can see this metric-size tire is basically the equivalent of a 35×12.50R20. Not all tire sizes are manufactured in both metric and inch sizes, so learning how to read a tire size can be a valuable tool for anyone in the market for new tires.

Tire Calculators

Tire calculators can be found online, and there are even free apps available that will do the math for you. Important information such as the height, load rating, speed rating, etc., can also be found directly on the tire manufacturer’s website.

Check back for our next installment – Tires 102. In part 2, we begin to break down all the tire terminology and what the ratings mean.

Load Index