You can find your vehicle’s tire size in the owner’s manual or on the vehicle placard located on the driver-side door frame. If you know your tires are the original equipment (OE) tires that came with the vehicle, you can check the tires' sidewalls for the size. Regardless of where you find it, you will need to know how to read tire sizes first.
On top of width and diameter, there are additional numbers and codes representing the details of the tire to consider: factors like "load index" and "speed rating."
It might be helpful to get a general idea of the construction of a tire.
HOW TO READ TIRE SIZE
Let’s use a sample tire size to explain the meaning of each measure. One of the most common tire sizes is P205/55R16 91H.
TIRE TYPE
Many tires include a letter that denotes their type. In our example, “P” stands for “P-metric,” meaning it is a passenger tire. P-type tires are designed for cars, crossovers and light-duty pickups.
“LT” tires are for light trucks and can support ¾ and 1-ton pickup trucks, SUVs and full-size vans. LT tires utilize heavy-duty construction that makes them more resistant to extreme conditions and vehicle loads.
Temporary spare tires are indicated with a “T” and should only be used in an emergency. For example, your car gets a flat, so you switch out to your spare and only drive it far enough to get to your closest tire shop.
Sometimes you will notice there is no letter tire size code. This omission usually means that it is a “metric” tire that originates in Europe. Metric tires have the same dimensions as P-metric but have been developed using world-recognized European standards. We go into more depth about Euro-metric vs P-metric tires elsewhere.
TIRE WIDTH
Immediately following the tire type you will see three digits representing the tire width (or section width) in millimeters. This refers to the distance between the furthest points of the tire’s two sidewalls. A larger width indicates a wider tire. Our example tire has a width of 205mm.
ASPECT RATIO
After tire width, you’ll find a slash and a two-digit number. This indicates the aspect ratio, which gives the tire height. Aspect ratio specifically is the tire sidewall height as a percentage of the width. A high aspect ratio means that a tire has a larger sidewall while a lower one means a lower sidewall.
Our sample tire size has an aspect ratio of 55, which means that the sidewall height is 55% of the width.
TIRE CONSTRUCTION
After the aspect ratio, you will find a letter that denotes the tire construction type. This may be either “R” for radial or “D” for a bias ply construction. Radial tires are composed of plies that run perpendicular to the bead and the ground. Bias ply tires are made from plies that crisscross in layers. Passenger tires are commonly Radial tires.
RIM DIAMETER
The number after the tire construction type is the rim diameter. This refers to the size of the wheel the tire fits on. Rim diameters often reflect common wheel sizes such as 14, 15, 16 and 17-inch wheels.
Our example wheel has a rim diameter of 16 so it can only be mounted on a 16-inch wheel.
LOAD INDEX
The load index indicates the maximum load-carrying capacity of a tire. It is represented as a number following the rim diameter. This value has a corresponding load, in pounds, which can be found on the standardized load index table.
Our example tire size has a load index of 91, meaning it can support 1356 pounds at maximum inflation as per the load index table.
SPEED RATING
A tire size ends with a single letter to represent its speed rating. This indicates the tire’s maximum speed capability as determined by laboratory testing.
Our sample tire has a speed rating of “H” which means that it has been tested to safely reach and sustain speeds up to 130mph.
If you need help checking your tire size and reading the sidewall codes, drop by your nearest Discount Tire store. We’ll get you taken care of.
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