Tire size Explanation

How To Read Your Tire
May 28, 2015 – 10:24 am
Lawnmower tires - How to read

How to Read Your TireThe sidewall of a car's tire has important information that tells you almost everything you need to know about it. Our video quickly breaks it down for you:

Here's a more detailed explanation. In this example, the code is: P215/65R 15 95H:

(P) Service Description
The service description may not always appear on the tire, but it is important to know how it can affect your vehicle. If there is a "P" on the sidewall, it stands for "passenger car." This refers to the U.S. (P-metric) method of tire sizing. "LT" stands for Light Truck, "ST" is for Special Trailer and "T" stands for Temporary, which is primarily used for small spare tires. If a tire does not have a "P" or another letter in front of the numbers, it is considered a "Euro-metric" tire. A Euro-metric tire conforms to the European tire specifications, and often carries a different load index than a comparably sized P-metric tire. We'll discuss load index in more detail below.

(215) Tire Width
The first number in this series refers to the tire's section width, or the distance from sidewall edge to sidewall edge, measured in millimeters up and over the tire's tread. Generally speaking, the larger this number is, the wider the tire will be.

(65) Aspect Ratio
This number is the tire's aspect ratio, or its section height compared to its section width. In this example, the section (or sidewall) height is 65 percent of the section width. This number can be indicative of a tire's purpose. Lower numbers, like 55 or less, mean a short sidewall for improved steering response and better overall handling.

(R) Internal Construction
The "R" refers to radial construction, which has been the industry standard in passenger-car tires for more than 25 years. Prior to radial tires, most cars came with bias-ply tires, which had a crude construction that made for poor handling. Bias-ply tires (which use a "B" for their description) are still used for certain truck applications.

(15) Rim Diameter
This is the wheel (or rim) diameter, in inches, for which the tire was sized. Pay particular attention to this number if you plan on upgrading your wheel size. If your wheel diameter changes, you'll have to purchase a new set of tires that matches this new diameter.

(95) Load Index
A tire's load index is a measurement of how much weight each tire is designed to support. The larger the number, the higher the load capacity. This is one of the most important numbers on your tire. To find out what "95" means, you have to look it up on a Load-Carrying Capacity Per Tire chart. Ninety-five indicates a maximum weight of 1, 521 pounds. Remember that this is per tire, which means you have to multiply by four to get the total capacity for a complete set of tires. If the vehicle has its original tires, you can just refer to the doorjamb, which lists the maximum cargo capacity with passengers.

Some vehicles are equipped with "XL" tires. It doesn't mean that they're extra large, but it does mean that they are extra-load tires. The load index on these tires is much higher than a standard-load tire, which is why it is important to replace an XL tire with another XL tire.

Earlier, we discussed "P-metric" and "Euro-metric" sizing, and it's the difference in their load rating that can lead to confusion and potential trouble. For a given size, P-metric tires will have a load index that is one or two points lower than corresponding Euro-metric tires. So if your car came with Euro-metric tires, don't replace them with P-metric tires. You can, however, replace P-metric tires with equivalent-size Euro-metric ones because you gain load capacity that way.

Why is this important? Generally speaking, you don't want your replacement tires to have a lower load index number than the originals (again, as indicated by the driver's doorjamb or the owner's manual), particularly with high-capacity vehicles that ride on smallish tires, such as minivans.

Also, optional large-diameter wheels with lower-profile tires tend to have less load-carrying capacity because they contain less air. And it is the volume of air inside the tire, not the rubber itself or the wheel material that shoulders the load. The load index is especially important when shopping for a tire online, since some retailers do not specify whether a tire is P-metric or not.

(H) Speed Rating
The speed rating is a measurement of the speed at which the tire is designed to run for extended periods. An "H" speed rating signifies that this tire can be run safely at speeds of up to 130 mph for extended periods. Will it explode if it goes to 140? Not immediately. But it might, if it is run at that speed for an extended time.

Here is a complete list of the various tire speed ratings, and their associated letters:
S = 112 mph
T = 118 mph
U = 124 mph
H = 130 mph

Source: www.edmunds.com
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