Tire Size Information appears on the sidewall of your tires. Look at your car's tires and find the numbers that are similar to the ones shown above. Specifically, find the "Width of tire", "Ratio of height to width", and "Diameter of wheel" numbers on your car's tire.
Want to know what all the other letters and numbers mean? For a better understanding of tire sizes, see the following examples or watch this video:
"P" indicates passenger vehicle tire. This includes cars, minivans, sport utility vehicles and light duty pickup trucks (typically 1/4- and 1/2-ton load capacity). Most vehicles can take P or non-P type tires. On AutoSquad.com, you don't need to worry about this because we take care of showing you tires that fit on your vehicle.
When there is no letter at the beginning of the size it denotes a "Metric" size (also called "Euro-metric" because these sizes originated in Europe). These are primarily used on European cars, but they are also used on vans and sport utility vehicles.
"LT" signifies the tire is a "Light Truck-metric" size that was designed to be used on vehicles that are capable of carrying heavy cargo or towing large trailers. This includes medium and heavy-duty (typically 3/4- and 1-ton load capacity) pickup trucks, sport utility vehicles and full-size vans.
Following the letter(s) that identify the type of vehicle and/or type of service for which the tire was designed, the three-digit numeric portion identifies the tire's "Section Width" (cross section) in millimeters.
Width: the 235 indicates this tire is 235 millimeters across from the widest point of its outer sidewall to the widest point of its inner sidewall when mounted and measured on a specified width wheel. This measurement is also referred to as the tire's section width.
Aspect Ratio: the 40 indicates that this tire size's sidewall height (from rim to tread) is 40% of its section width. The measurement is the tire's section height, and also referred to as the tire's series, profile or aspect ratio. The higher the number, the taller the sidewall; the lower the number, the lower the sidewall. Many of today's tires have a "low profile" which is indicated by a low number.
A letter (R in this case) that identifies the tire's internal construction follows the two digits used to identify the aspect ratio.
The R in the P235/40R17 size identifies that the tire has a Radial construction
If the R in the size was replaced with a D (P235/40D17), it would identify that the tire has a "bias ply" construction. If the R in the size was replaced with a B (P235/40B17), it would identify that the tire is "Belted." Tires using this construction are practically extinct.
Today, the only tires that continue to include the speed rating "in" the tire size (P235/40ZR17) are Z-speed rated tires. In this case, following the two digits used to identify the aspect ratio are the letters ZR to identify the tire's speed rating (Z) and its internal construction (R). Since 1991, all other speed ratings are identified in the tire's Service Description (which will be covered shortly).
Diameter: the 17 indicates the tire and wheel diameter designed to be matched together.
Tires that have a rim diameter expressed in inches (P225/50R16, as well as 8, 10, 12, 13, 14, 15, 17, 18, 19, 20, 22, 23, 24, 26 and 28) are called "inch rim" sizes, are the most common type of tire size and are used on most cars, minivans, vans, sport utility vehicles and light duty light trucks.
The 91S represents the tire's Service Description. A Service Description identifies the tire's Load Index and Speed Rating. Service Descriptions are required on all speed rated (except for Z-speed rated) tires manufactured since 1991.
The first two digits (91) represent the tire's load index and are followed by a single letter (S) identifying the tire's speed rating.