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How to Read Tires


Understanding Tires
Thetires are the four small patches of rubber that connect your car to the road. So, the more you understand about what is keeping your car running, the better you will be at maintaining it’s health. Also, visiting a tire service in Houston will help you to keep up your tires for long. Let’s understand what those numbers and letters mean on a tire.

Tire Sidewall Example: P225/55R18 97T

Tire Chart

Tire Type

Most tiresizes begin with a letter or letters that indicate the vehicle and/or service they were designated for. Here are the most common ones.

  • P: The letter “P” indicates that the tireis a “P-Metric” tire, which means that the tiresare made to US standards, intended for passenger vehicles.
  • None: If there are not any letters at the beginning, then it is a “Metric” or “Euro-metric” tire. These tiresare typically made for European cars, but they are also sometimes used on vans and SUVs.
    • “P-Metric” and “Euro-metric” tiresare dimensionally the same, but their load carrying capacities may vary a bit.
  • T: The letter “T” indicates “Temporary Spare”, meaning that the tirewas meant for temporary use until the flat tire can be repaired or replaced.
  • LT at the start: If the letters “LT” are at the beginning it means the tireis a “Light Truck-metric” tire and is meant for vehicles that are used for carrying a heavy load or towing large trailers.
  • LT at end: If the letters “LT” are at the end (e.g.“7.50R16LT 112/107Q”) then the tireis an earlier “Numeric”, “Wide Base”, or “Flotation” Light Truck sizemeant for vehicles used for carrying heavy cargo or towing large trailers (Numeric), use 16.5 inch diameter rims (Wide Base), and helps the vehicle drive on loose dirt or sandy surfaces (Flotation).

Tire Width

The three digits you read first are the tirewidth, or section width (also known as the cross-section). This is the nominal width of the tiremeasured in millimeters from the widest point of the outer sidewall to the widest point of its inner sidewall. In the above case, the width is 225 millimeters.

Aspect Ratio

The two-digit number that comes after the slash mark is the aspect ratio in millimeters. In the above case, the aspect ratio is 55.The number indicates the ratio of the height of the tire’s cross-section to its width. So, 55 would mean that the height is equal to 55% of the tire’s width. If the aspect ratio is large, the tire’ssidewall will be large as well.


The letter that comes after the aspect ratio is the tire’sinternal construction type.

  • R: The letter “R” indicates “Radial” which means the layers run radially across the tire, particularly perpendicular to the axis of rotation. While there are many other construction types, radial tiresare the most popular.
  • D: The internal construction plies crisscross on a diagonal and therefore has “bias ply” construction. These tiresare usually made for light truck and spare tires.
  • B: This would mean that the tirehas a diagonal construction like the “D” tires, but also are reinforced with belts under the tread area, hence the name “Belted.” Tires are hardly ever made in the way anymore.

Wheel Diameter

The number that comes after the construction type is the wheel diameter which is 18 in the above case. It indicates the size of the wheel, in inches, the tire is meant to fit, measured from one end to the other. So, our example has an 18-inch diameter. Tires will typically range from 8 to 28 “inch rim” sizes. These sizes are the most common, usually used on cars, vans, SUVs, and light duty trucks.

There are 2 other diameter indicators, but they are uncommon. The first is a rim diameter expressed in “half” inches, for instance, 16.5 or 19.5. These are usually found on tire on heavy-duty trailers and light trucks, and box vans. The other indicator is expressed in millimeters, called millimetric sizes. These numbers will be in the hundreds, such as 365 or 415.

It is important that you do not combine tires and wheels that have the common rim diameters with tires and wheels that are in half inches or millimetric sizes. Please make sure you regularly visit a shop for tires in Houston to check that the tire and wheel diameters match exactly before you mount the wheel.

Load Index

The last two numbers in the sequence, before the very last letter, is the load index, which describes the maximum load the tire can carry when fully inflated. The load index refers to a chart that will translate the number on the tire to a load in pounds or kilograms. The higher the load index number, the more weight the tire should be able to carry. In the above case, 97 would mean that our tires can carry 1609 pounds or 730 kilograms.
The index starts at 1 and goes up to 150, representing 99 to 7385 pounds.

Speed Rating

The last letter in the entire series indicates the speed rating for the tire, which is the speed capability of the tire. The letter refers to a chart which will tell you the speed capability of an undamaged and properly inflated tire in miles per hour and kilometers per hour. In above example, “T” would indicate a maximum speed capability of 118 mph, or 190 km/h, a tire usually meant for family sedans and vans.
Despite the maximum speeds listed, manufacturers and tire shops in located in the Houston area highly recommend that you always follow the legal speed limits.

DOT Symbol:

“DOT” on the sidewall indicates that your tire complies with Department of Transportation safety standards. The 3 sets of letters and numbers that come after it indicate the tire’s identification number. It tells the tire’s manufacturing location, manufacturer’s code, and the week and year the tire was manufactured.

UTQG Code:

UTQG stands for Uniform Tire Quality Grading. Established by the National Highway Traffic Safety Administration (NHTSA), the code indicates the tread wear, traction, and temperature tolerances of the tire. Tread wear refers to the wear rate of the tire, comparable only to other tires in the manufacturer’s line. 100 is the baseline, so a tire with a grade of 200 would like twice as long during government testing. Traction indicates the traction capabilities on a wet road.
The grades go from AA, the highest to C. Temperature grades indicate how well the tire can withstand and dissipate heat. A higher grade means that the tire can operate at faster speeds. The grades go from A to C, with A specifying that the tire can run at faster speeds.

If you are having trouble reading tires don’t worry, our technicians from our Houston, Texas tire shop can easily assist you with this. Typically, we will have you take a picture of the tire, and text it or send it via our automated uploading system so that we can make sure that your tires are correctly sized.


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