Tire Codes - What Do They Mean?
Tire Codes - What Do They Mean? There’s a lot of important information on the sidewall of a tire, but deciphering all of it can be tough. By understanding these codes, you can decide if a tire is right for their vehicle, driving style, and road conditions. According to US News & World Reports, some of the information is mandated by government regulation, providing a description of the tire’s size, intended use, capacities, and manufacturing source.
Brand and Model
The largest type on the sidewall shows its brand and model. That might seem like basic information, but if you are shopping for a used car, you’ll want to walk all the way around to make sure that all of the tires match. If you are replacing your vehicle’s tires, consult your car’s owner’s manual to determine its minimum tire specifications. Don’t rely on the numbers on the existing tires, as those tires may be replacements that don’t match the vehicle’s needs.
Width of the Tire
If there are letters, they’re likely a P that indicates that it is a passenger car tire or LT to signal it is a light truck tire. The first three-digit number is the tire width in millimeters measured at its widest part, not the tread. High-performance car tires tend to have higher section width than more mainstream vehicles.
Height of the Tire’s Sidewall
The two-digit number following the slash in the tire size description is the tire’s aspect ratio, the sidewall height expressed as a percentage of the tire’s width. If the markings say 255/55R18, it means that the height of the sidewall is 255 multiplied by .55, or 140 millimeters. The higher the aspect ratio, the taller the sidewall is in relation to the tread width. If it is a low number, it means that it is a low-profile tire such as those found on higher-performance cars. Passenger vehicles equipped with tires featuring higher aspect ratios generally have a softer ride than models with low-profile tires.
Size of the Wheel It Fits
The next part of the tire size is a letter denoting its construction, sometimes a Z speed rating, and the size of the wheel that it is designed to fit, measured in inches. When it says R18, it means that it has a radial-ply construction and it fits an 18-inch rim. A tire can only be installed on a wheel that is identical to its inner diameter.
Maximum Load the Tire Can Carry
The next set of letters and numbers are a tire’s service description, which tells you its load index, or how much weight it is designed to carry. You can find the recommended tire load range for your vehicle by looking at the tire placard on the pillar behind the driver’s door or the vehicle’s owner’s manual.
The final letter in the service description is the tire’s speed rating, or the maximum speed it is certified to safely be driven at when properly inflated and maintained. Speed ratings range from L (75 mph) to Y (186 mph). When you are replacing tires, the new tire’s speed rating should match or exceed the value listed in the vehicle’s owner’s manual.
Elsewhere on the tire there will be a block of numbers and letters known as a DOT Code (Department of Transportation Code). The first part is a batch and plant identifier that can help you in the case of a recall, but it is the last four digits that are more important. The final four numbers show the date that the tire was manufactured. The first two digits are the week and the second two are the year. It’s especially important to look at the date if you are considering a used car or your car gets limited use each year. While tires don't have an expiration date, tires older than five years should be carefully inspected, after 10 years they should definitely be replaced.
Maximum Inflation Rating
Another number you will find on the tire is its maximum inflation pressure. It is not, however, the pressure that you want to inflate your tires to. Instead, you should consult your car’s owner’s manual or tire placard for the recommended tire pressure. Inflating your tires to maximum pressure will create uneven tire wear, change the car’s handling and comfort, and potentially compromise its stability control and advanced safety systems.
Uniform Tire Quality Grading / Treadwear
The Uniform Tire Quality Grading (UTQG) marking is designed to show a tire’s treadwear rating, temperature range, and traction ability. The treadwear test uses a reference tire with a rating of 100 as a baseline. A tire with a rating of 300 would be expected to last three times as long. A tire with a treadwear rating of 640 could be expected to last 6.4 times longer than the reference tire.
Uniform Tire Quality Grading / Temperature and Traction
Temperature ratings are a measure of a tire’s propensity to create heat and its ability to dissipate it. A rating of “A” is the best and “C” is the lowest. All tires sold in the U.S. must earn at least a “C” grade. A tire’s traction grade measures its coefficient of friction or braking g force as it is forced to skid across wet concrete and asphalt surfaces. A score of “AA” is the best a tire can earn, while a “C” is the lowest. The test does not evaluate a tire’s tread design or its ability to resist hydroplaning.
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