Tire Size Guide | Tire Size Explained, Understanding Tire Letters

A tire’s sidewall is filled with important information, but you may not know how to read it. When it comes time to replace your tires; however, you need to understand the letters and numbers of your tire size. It may seem like it is written in code, but don’t worry, RNR Tire Express - Midwest can help you decipher your tire size with this breakdown of what it all means.

Understanding Tire Letters and Numbers 

Example: P 225 / 45R17 91V

P or LT = Service Type

Most tire sizes begin with one or more letters—for example, P or LT—which indicate what type of vehicle the tire was designed for. 

P stands for P-Metric, which is the most common type of tire. These tires are designed for use on passenger vehicles like cars, minivans, light-duty pickup trucks (1/4-ton or 1/2-ton capacity), and SUVs. 

LT identifies Light Truck tires. These tires are designed for vehicles used to tow trailers or carry heavy loads including SUVs, full-size vans, and medium-duty and heavy-duty pickup trucks with 3/4-ton to 1-ton capacity.

225 = Section Width

The three digits following the service type tell the section width, which is the measurement from sidewall to sidewall in millimeters. The section width can be converted to inches by dividing the width in millimeters by 25.4 (there are 25.4 millimeters per 1 inch).

45 = Aspect Ratio of Sidewall

The two-digit number that follows the tire's section width tells the aspect ratio. It is a percentage of the section width. In our example, the 45 indicates that the sidewall distance, from the wheel rim to the outside of the tread, is 45% of the section width. A lower aspect ratio means a lower-profile tire with a shorter sidewall, while a tire with a higher aspect ratio will have a taller sidewall and look more like a donut.

R = Internal Construction

The letter (R, D, or B) following the two-digit aspect ratio tells the tire's construction. R stands for radial construction; D stands for diagonal bias construction; B stands for belted bias. R means that the internal body plies of the tire radiate outward from the center—over 98% of all tires sold today are radial. D means that the internal body plies of the tire crisscross on a diagonal pattern. B, which is rarely seen these days, means the internal plies crisscross and there's also an extra layer of reinforcing belts under the tread area.

17 = Wheel Diameter

Wheel diameter specifies the size of the wheel rim in inches. In our example, the 17 means that the tire should be matched to a 17-inch diameter wheel. Tires usually come in the following widths: 8, 10, 12, 13, 14, 15, 17, 18, 19, 20, 22, 23, 24, 26, and 28. Tires in these sizes are typically found on most passenger cars, light-duty light trucks, SUVs, minivans, and vans. 

91V = Service Description Rating

Following the tire size is the load index and speed rating, or service description, which is set by the manufacturer. This information is important because you should never replace your tires with a lesser speed and load rating than the original equipment tires. 

The load index tells the maximum weight the tire can support when properly inflated. It ranges from 75–105 for passenger tires, with each numeric value corresponding to a certain carrying capacity (see chart below). In our example, the load index is 91, which means the maximum weight capacity is 1,356 pounds.

Speed ratings are represented by letters ranging from A to Z and designate the maximum speed a tire can be driven on under its recommended load capacity (see chart below). In our example, the speed rating is V which means it can be driven at speeds up to 149 mph. Even though a tire can perform at this speed, we do not advocate exceeding legal speed limits.

Tire Load Index Chart

Tire Speed Rating Chart

Find a Tire Store Near You

When it’s time to shop for tires, turn to RNR Tire Express - Midwest for the best brands on the market. We offer affordable, quality tires with no credit check and flexible payment options! Visit us online or find a store location today!