Buying Tires and What Automotive Tire Codes Mean
Tire Size Coding for Selecting the Right Tires for a Car or Truck
Aug 25, 2009
Tires have been standardized to meet certain requirements and a system of coding tires has been created to identify each tire, not only for its size but also for the type of tire and speed or load ratings.
Most tire dealerships will choose an adequate tire for a vehicle that they are performing installations on, but some will sell the best tire for the car based on their profit, not for what is truly best for the vehicle. Learning to understand the manufacturing code system is necessary for car owners who want to be sure that the tires they are buying are the best for their vehicle.
Automotive Tire Size Coding and What it Means
Imprinted into the sidewall of every tire is a code that describes the size, type and ratings of each tire. The general coding looks something like, P215 / 70 R14 82S though the individual numbers and letters will vary with size and type. The first letter, "P" signifies that it is a passenger car tire–if it were for a light truck it would be "LT". The next set of three numbers is the width of the tire in millimeters, so the larger the number, the larger the tire width.
The next set of numbers is the percentage that the sidewall height is to the width of the tire. A 70, as in this case, means that the height of the sidewall is 70% of the width of the tire. So a low profile tire would be something more like P245 / 45 ZR17 this would mean that the height of the tire is only 45% of the width, which in this case is much larger than a standard passenger tire size.
The next letter or combination of letters signifies what kind of tire it is. An "R" by itself indicates that it is a radial tire. Before 1991 a ZR meant that it was rated for a high rate of speed since the Z indicated the speed rating. After 1991 the speed rating was moved to the end of the code, but many manufacturers still include the Z combined with the R in their high performance tires.
The next two numbers are the rim size. This is a non-negotiable number. Some of the other numbers can be changed a little depending on what kind of look or performance a driver wants their vehicle to have, but if a rim is a 14" rim, only a 14" tire will fit on it.
Speed and Load Ratings for Automotive Tires
The last two numbers are the load rating which is more useful with truck tires than for passenger car tires. If a light truck is going to be carrying heavier loads often, then the truck owner would most likely want to purchase tires that are rated higher for heavier loads. A load rating is not required to be imprinted on light truck or passenger tires, but it is sometimes included by the manufacturer.
A letter in the next position indicates the speed rating. It is not required to be imprinted on all tires but it can be found on many. The average passenger car tires are speed rated with an S (113 mph) or T (116 mph). The rating indicated the highest recommended speed for that tire. The Z rating in our example is for a tire that is rated at over 149 mph, which deems it to be a high-performance tire.
Just about anything that a car owner needs to know about their tires can be found by reading the sidewall. Drivers should also remember that the tire inflation pressure is also marked in PSI, pounds per square inch, on the sidewall and the tire pressure should be checked frequently to be sure that the car is running on properly inflated tires. This will help to ensure a longer life for the tires and a safer ride for those in the vehicle.
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