How to Read Tire Sizes
It is imperative to the overall quality and performance of your vehicle to install the proper tires and maintain the right air pressure. Each tire has a series of numbers and letters which indicate if it is right for your vehicle, as well as the wear and tear it can handle. In this article, we will explain what those numbers mean and how they will help you select the right tire for your vehicle.
Finding the tire size for your vehicle
Other than reading the size on the tire’s sidewall, you can check the owner’s manual that came with your vehicle. If you do not have this, then check the driver’s side door frame, the glove box door or the gas tank cover.
Decoding the numbers on your tire
The numbers on a tire provide more information than just the size of the tire. These numbers provide the manufacturer’s brand logo, speed rating, how much air pressure is needed, and more. Here is an example of what appears on the tire’s sidewall:
P 215/65R15 95H M+S
Keeping this example in mind, we will go over what each of these letters and numbers means.
(P) - The First letter indicates the vehicle classification for the tire used. In our example, the number sequence starts with a P. P means that it is a passenger car tire. If you drive a small truck, the letters at the beginning of the sequence would be LT, which stands for a light truck. If it is a European car, then there will be no letter at the beginning of this series.
Section width/Aspect ratio
(215/65) - These numbers are a combination of measurements. The section width is the first number (215). It is measured in millimeters and goes from one sidewall to the other sidewall. The aspect ratio is the second number (65). Aspect ratio measures the height of the sidewall. In our example, the height is 65% of the section width.
(R) - This letter indicates how the tire is designed internally. For instance, the tire in our example is a radial tire; which means the rubber has cords that run from side to side throughout the tire. The layers of cord and rubber strengthen the tread, so it is safer and lasts longer. Other letters you may see are D and B; these are not as common as radial tires. D stands for bias ply, meaning the layers of cords crisscross on a diagonal pattern. If a tire has belts, it is labeled with a B and has crisscrossed cords just like a biased tire, but also sports a layer of belts to reinforce the tread.
Motorcycle tires contain the letters B (for "belted") or R (for "radius"). If there is no letter present, it is a biased tire.
(15) - Represents the size of the wheel or rim the tire was made to fit. This tire was made to fit a 15-inch diameter wheel. Most tires have inch rims, which means the wheels are measured in inches. These are the most common tires; however, there are unique rims that are measured in half inches. These are normally designed for heavy-duty commercial vehicles and trailers.
(95) - The load index specifies how much weight the tire can transport safely. The tire must be installed and inflated properly to carry the specified amount of weight. The manufacturer indicates load index specification your tires should meet this requirement or exceed it to ensure safety.
A load index of 95 means that your tire can safely carry 1,521 lbs of weight. Multiply that number by four – four tires to a car – to get your car's maximum carrying capacity. When replacing tires, never install tires with a load index below what came factory-installed on your vehicle.
The speed rating indicates the maximum speed a tire can withstand safely. Normally, these ratings match the highest speed the vehicle can go. Since 1991, all tire manufacturers are required to put the speed rating at the end of the number sequence next to the load index. There is a difference between spare tires, non-performance tires, and performance tires.
- Non-performance tires have the letters P, Q, R, S or T. Spare tires will have M or N next to the load index.
- Performance tires are for sport or high-performance cars. In this example, the letter Z indicates it is for a sports car. All performance cars also have the letters U, H, V, W or Y next to the load index to indicate the maximum speed the tire can withstand. This example shows the letter W meaning this tire can safely go 168 mph.
Department of Transportation (DOT) serial number
All tires must pass the Federal Motor Vehicle Safety Standards in the United States before being approved for marketing. This is indicated with the letters DOT, then followed by a series of numbers called the Tire Identification Number (TIN). The TIN tells which factory manufactured the tire along with the week and year. An example is DOT R8LN LMJR4610. This number means it was DOT approved, R8 is the manufacturer, LN is the size or plant. LMJR determines the construction, tread pattern, and category. 46 is the week and 10 is the year the tire was manufactured.
Uniform Tire Quality Grading (UTQG) Marking
The DOT developed this rating system to let consumers know what grading the tire received during testing. All tests are performed by the manufacturer and not the DOT. This grading system allows you to choose the right tire for your driving preference. Quality grade markings tell you the type of traction the tire has in ice and rain, how this the tread is, and what temperatures the tire can withstand.
Example: Tread 440, Traction A, Temperature Rating A.
Treadwear does not necessarily rate how long the tread will last, but it does tell how durable the tread is.
Traction determines how well the tire can stop on a wet road and stay in a straight line. This does not determine how the tire takes corners or if it will hydroplane. In this example, the traction rating was A; meaning the tires was found to have the second-highest grade. The grading table from best to worst is AA, A, B, and C.
Temperature Rating grades the tire's ability to withstand heat when driven in high temperatures. The rating is graded from A to C, with C being the poorest rating.
Maximum inflation pressure and maximum load
The maximum inflation pressure numbers indicate the how much air the tire can hold when the vehicle is in use. Before inflating your tire, check the manufacturer’s recommendations in the vehicle’s owner’s manual. Never add more air that what is recommended for your vehicle.
The maximum load number on the tire tells you the maximum weight the tire can carry. Like with the inflation pressure, always refer to your owner’s manual to determine the manufacturer’s suggestions.
Understanding motorcycle tire sizing
Motorcycle tires are similar to car and truck tires, although there are different sizes of tires for different types of bikes.
Tire dimensions for motorcycles are as follows:
- Metric – the most common tire used on sport bikes, cruisers, and touring bikes, as well as others. For tires with no speed rating, then M will be before the metric size, followed by radial or bias, then the wheel diameter.
- Alpha – used for touring bikes. These tires can be belted even if there is not a B after the speed rating.
- Standard inch – these are older motorcycle tires and do not provide an aspect ratio. The tire height is 100% of the width. These are biased tires and are not belted. Standard inch tires cannot be used on newer motorcycles.
- Low profile inch – these always have an 82% aspect ratio, unless it is indicated on the tire as 85%. These tires are rarely ever used.
Other motorcycle tire identifying letters are
- WW – this indicates a white wall tire
- TT – indicates there is an air tube on the inside (Tube-type Tire)
- TL – means there is no tube inside the tire (Tubeless)
- M/C – means Motorcycle – these can only be used on motorcycles
Buying new tires seems like it should be a simple task. However, each vehicle requires a specific type of tire depending on how the vehicle is used and the wear and tear it will sustain. Abiding by the guidelines provided by your vehicle’s manufacturer and the tire’s manufacturer will ensure your safety. For more information on tire sizes and how to determine what is best for your vehicle, ask an Expert.