ALT-1 Automotive Maintenance a Guide to Tire Sizes
Deciphering tire sizes can be confusing. It mixes the metric system with the American measurement system and it's not exactly intuitive, but it's not quite as difficult as it might seem at first. There are two primary methods of sizing tires: metric sizing and flotation sizing.
Let's look at an example of metric sizing. Metric sizing is typically displayed in this format: 265/70HR17. This will be preceded by a P, which designates that the tire is for a passenger vehicle, or an LT, which designates that the tire is for a light truck. The 265 in this example represents the tire width in millimeters. The second number, 70, is what's know as the aspect ratio. It's the height of the tire from the top of the rim to the top of the tread represented as a percentage of width. In other words, the tire is 70% as tall as it is wide. The H between the last two numbers refers to the speed rating of the tire. This is the highest speed at which a tire can safely travel for an extended period of time. The last number is the rim diameter in inches.
It's easy to see how this might be confusing with the switch between millimeters and inches and the height of the tire being expressed as a percentage of width, but here's what you need to remember: The first number is the width, the second number is the height, and the third is the rim diameter. If you have two tires, a 265/70HR17 and a 265/65HR17, the tires are the same width, but the 265/70R17 is taller.
Flotation tire sizes are much easier to understand, however they are far less common. You typically only see flotation sizes on off-road vehicles and larger trucks. For example, if a vehicle has size 30X9.50R15 tires on it, the first number is the overall tire diameter in inches, the second number is the tire width in inches and the third number is the rim diameter in inches. This is a far easier method of tire measurement to understand, unfortunately, it is also rarely used anymore except in specialty applications.
Hopefully, this clears up some confusion about tire sizing. It may be a bit complicated at first, but fortunately the tire industry has at least evolved into using a standard sizing format for most vehicles. This makes purchasing replacement tires quite a bit easier.
