Guide to Tire Size Markings and Tire Markings
Markings on American Tires
In the United States, every tire is required by federal law to receive approval by the Department of Transportation (DOT) before entering the market. Accepted tires meet the minimum standards for safety. This process involves receiving unique markings to denote size, age, manufacturer and other criteria to inform the consumer about where their tires came from, and whether or not they need replacing because they’re growing too old. All drivers must be made cognizant of the required tire markings in order to maximize their chances of staying safe while on the road.
Tire Size Markings
In the image below, the tire size is listed as 185/60R14. Markings such as this break down in the following manner:
- 185 indicates section width in millimeters
- 60 indicates aspect ratio of the sidewall’s height to the tread’s width
- R indicates the construction type of the tire, which, in this example, is radial
- 14 indicates the size of the rim and rim
How Uniform Tire Quality Grading Marking Works
By law, all tire manufacturers are required to grade their tires under the DOT’s Uniform Tire Quality Grade (UTQG) labeling system. Temperature resistance, treadware and traction comprise the testing rubric, which is applied independently on a manufacturer-by-manufacturer basis. These involve assigning values to the federal tire safety guidelines and comparing the test tire with a control. It is important to note that treadware ratings are set by the manufacturers rather than the DOT, so only temperature resistance and traction get held to a standard performance level. Because of this, treadware numbers are best compared to other tires produced by the same company.
Tire Markings Indicating Tire Size
When trying to glean a tire’s size, look for the markings on its sidewall. In this example, we’ll break down P185/60R14 82H.
- P indicates that you’re dealing with a passenger, or car-type vehicle, tire. If there is no P, which means metric, or LT, which stands for “light truck” for heavier, larger loads and more demanding environments.
- 185 here notes the width of the tire in millimeters, with the high numbers denoting the wider tires.
- After the slash comes the aspect ratio, which is the sidewall’s height in relationship to the overall tire width. For example, an aspect ratio of 50 means the tire sections are half as tall as they are wide. The shorter the sidewall, the easier the vehicle will be to handle, while taller sidewalls tend to deliver smoother drives. Depending on the tire width, shorter sidewalls are noted by smaller numbers.
- R indicates that the tire is of radial construction. Other options for this space include D for diagonal/bias and B for belted bias.
- Wheel size designation indicates the radius of the tire’s hole. Wheel diameter and tire diameter HAVE to match for maximum safety. Only 15” rims should support 15” tires – and vice versa. Failing to properly align the two can place drivers and passengers at an increased risk of serious injury or death as a result of blowouts, tread separations, roof crushes and rollovers.
- Service descriptions are comprised of speed symbols (a letter) and load indices (a number). The latter needs to be either equal to or exceeding the load index of the previous tire, because it indicates its weight-carrying capacities. Again, higher numbers mean a higher capacity for load-carrying. When replacing your tires, you must always choose a model that either meets or exceeds this number, as they have to be able to support the weight of your car.
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