How To Understand Car Tire Sizes

Your tires have a set of numbers about an inch high embossed on the sidewall. The numbers on your tires represent specific characteristics that help you select the right one for your vehicle and the way you drive. These numbers provide a great deal of information about the tire as long as you know how to read them.

1. Look at the first letter. It designates the tire’s intended use. P is very common; it stands for “passenger” and is the norm for most cars and for pickups up to 1/2- ton load capacity. Heavier pickups will use the LT or “light truck” tires. A T designates a temporary spare. A tire with no letter at the beginning is a metric tire for European vehicles. ST tires are designed for trailers and should not be on automobiles at all.


2. Picture yourself inside your tire. The rest of the numbers can be difficult to understand. To make things clearer, imagine that you are standing behind your vehicle and one of the tires expands to the size of a spaceship. Now mentally move yourself, without turning, inside the tire. There is tread below, the wheel overhead and sidewalls to each side.


3. Hold out your imaginary arms, still inside the tire. That first number, 235 or 195 in the examples, is the distance along your arms from the outside of one sidewall to the outside of the other, expressed in millimeters. It is called the section width.


4. Get out a calculator. The second number is the sidewall aspect ratio. The height of the actual sidewall, from the tread up to the edge of the hole in the center, divided by the section width and multiplied by 100 yields the aspect ratio. Smaller numbers mean stubbier sidewalls.


5. Read the single letter after the aspect ratio. This designates the tire’s construction. The norm today is R for radial, but D for diagonal and B for belted, also occur.


6. Find the number after the letter. It represents the size of wheel the tire is meant to be used on, from 8 inches to 28 inches. Heavy-duty trailers may use sizes with half inches, as 17.5 inches and some manufacturers express this number in millimeters.


7. Look at the final number. This represents the load capacity, a somewhat arbitrary designation that tells how much weight the tires are designed to carry. Read the number and look it up in a chart, either on the Internet or at your preferred tire shop.


8. Compare the letter at the very end, the speed rating, with a chart, again either on the Internet or at a tire shop. These designate the speed at which the tire can safely operate. They go from Q at about 99mph or 160kph up to V at about 130mph/210kph, with a catchall Z rating for tires designed for even faster speeds.


9. Not every combination of letters and numbers will be available. LT tires designed for heavy pickups are unlikely to need the speed capacity of Z rated tires, for instance. There is enough variation, though, to allow a smart shopper to select an appropriate tire for almost any use.