Are you ever left confused and speechless when you are confronted with the question of "what are is your tire size?" It sure is confusing with all these numbers and letters in a weird foreign code of which you have not been taught in. But don't worry, our tire sizing guide will decode this foreign language for you!
Look for these numbers on your tires to determine the specifications of the tires.
The P at the beginning tells us the tire type, referring to the manufacturing standards of the tire, made for specific classes of vehicle. P = Passenger Vehicles (ie cars, minivans, light-duty pickup trucks and SUVs) LT (code begins with LT) = Light truck (For vehicles used to tow trailers or carry heavy loads; SUVs, full-size vans, medium- and heavy-duty pickup trucks) LT (code ends with LT) = Refers to the tire as either a "Numeric", "Wide Base" or "Flotation" Light truck size (For vehicles capable of carrying heavy cargo and towing trailers) ST = Special Trailer (To be used on car, boat or utility trailers) T = Temporary (Temporary spare tires designed for short-term use until the regular tire is repaired/replaced) C = Commercial (For use on delivery trucks and vans capable of carrying heavy loads)
If there are no letters in the front of the numbers, don't worry because it only means that they are Metric (also known as Euro-Metric) tires. These tire sizing originated in Europe and are equivalent to the P-Metric (P-metric, as shown above, are the most common tires) sizes in the dimensions. Metric tire sizes have very subtle differences in the load-carrying ratings and capacities. The Metric sizes ending with a C refer to the Commercial tires.
The 3 numbers after the initial letter (if applicable) refer to the tire's section width. It is the cross section measurement of the tire in millimeters. For the above example, it is a P185/75R14 which means it is a passenger tire and the width of the tire is 185mm wide. This measurement is measured from the widest point of the outer sidewall to the widest point of the inner sidewall of the tire when properly mounted.
The 2 numbers after the section width refer to the tire's profile/aspect ratio. It is a ratio determined by the height and width of the tire. In the above example P185/75R14, the aspect ratio 75 indicates that this specific tire size's sidewall height (from the wheel rim to the tread) is 75% of its section width (recall; section width is the cross section measurement/width of the tire). So what does this mean anyways? Basically, the higher the number for the aspect ratio is, the taller the sidewall and the lower the number, the lower the sidewall.
The single letter after the 2-digit aspect ratio will tell you about the tire's construction. In the above example P185/75R14, the R stands for radial. Most tires today have radial construction. This means that the internal body plies of the tire radiate outward from the center. There is also D, which refers to a bias ply construction, where the internal body plies of the tire crisscross on a diagonal pattern. Tires using this construction are for light truck and spare tires. If there is a B, referring to a belted tire, the internal plies crisscross in the same manner as a D construction, but there's an extra layer or reinforcing belts under the tread area.
Tire and Wheel Diameter
The 2-digit number following the internal construction letter refers to the diameter of the wheel to be matched to this certain tire. In the above example P185/75R14, a 14-inch wheel needs to be used with the tire for proper fitment. Sometimes this number may be displayed in millimeters, simply convert the number by dividing by 25.4 to obtain the sizing in inches. There may also be rim diameters expressed in half inches - mostly for heavy-duty trailers, heavy-duty light trucks and box vans.
Load and Speed Rating
The 2-digit number followed by a single letter refers to the load and speed ratings respectively. Collectively it is also referred to as the Service description In the above example, P185/75R14 82S, 82 refers to the load rating and S is the speed rating. The load rating is used to to tell you how many pounds the tire can safely carry. Now let me reassure you that the number for the rating shown does not correspond to the amount of weight it can carry (ie 82 does not equal 82 pounds or 82 kilos). There is a handy reference list. It is very important to maintain a proper load index for your vehicle when you replace your tires. The speed rating ensures the tire will match the vehicle's top speed capability. It is necessary to use a tire that will match your vehicle's top speed capability. Do not use tires with different speed ratings as it will affect your handling. Generally tires with higher speed ratings will help get rid of heat better as they have higher quality construction and ply. This will allow for better cornering, gripping and braking.
Typically passenger vehicles have a load index ranging from 70 to 110.
Load Rating Chart
Speed Rating Chart
Maximum Speed (mph)
Maximum Speed (km/h)
Off-Road & Light Truck
Heavy Duty Light Truck
Family Sedans & Vans
Family Sedans & Vans
Sedans & Coupes
Sport Sedans & Coupes
Exotic Sports Cars
Exotic Sports Cars
Exotic Sports Cars
And there you have it - now you know how to decode tire specifications!
There are also other important information you can gain from reading the side wall of the tire, such as the date the tire was manufactured, and if you want to learn how to decode that information - stay tuned!
The next time someone asks you for your tire size, just remember the code goes by: Type, Width, Ratio, Construction, Rim Diameter and then Load and Speed Rating.