Marc Christ Associate Editor
February 1, 2013

Tire Specs
If you look closely, tires are riddled with writing, numbers, and symbols. But what does all that gibberish mean and why is it important? Some of it is inconsequential to you, the consumer, but much of it is valuable information.

Other than the brand name and line name (almost like a make and model of a car), the other prominent indication on the sidewall of all tires is the size. This is usually the number one factor when considering tires, so it's important to understand how the metric size designation works. A 245/45-17 tire has a section width (sidewall to sidewall) of 245 mm. This is not tread width, but actual width from the outside edge to the outside edge of the sidewalls.

The "/45" (in the 245/45-17 tire) refers to the sidewall aspect ratio. In other words, the distance from the wheel's rim to the tread (the sidewall width) is 45 percent the width of the section width. And the -17 (or R17 or ZR17) is the wheel diameter. The "R" designates that the tire is a radial, and "ZR" designates that it is a Z-rated radial, the only speed rating shown in tire size designations.

So to compute tire size using the English measuring system, you divide 245mm by 25.4 (the conversion factor), you get the section width in inches (9.65). Since the sidewall is 45 percent of that, you multiply 9.65 x 0.45, which equals 4.34 inches. And since we know the diameter of the wheel to be 17 inches, you get 17 + 4.34 + 4.34 = 25.68 inches diameter.This is how tall the tire is.

Immediately following the tire size is the service description, which consists of the load index and speed rating. The load index is a two- or three-digit number, ranging from 71 to 110, and indicates how much weight the tire is capable of bearing. This is not typically an issue when dealing with passenger car tires, especially Mustangs.

The speed rating, though, is very important. If you are going to be competing in drag racing or road racing events, be sure to have tires capable of the speeds that you will be achieving. Most production coupes and sport sedans are equipped with H (130 mph) or V (149 mph) speed-rated tires.

Z-rated tires are designated by the "ZR" in the size designation and were originally meant to be rated for speeds in excess of 149 mph. However, higher-rated tires have since been developed, due to the greater speed capabilities of newer production cars. Speed rating W (168 mph) and Y (186 mph) tires are typically meant for exotics, but you can purchase these tires as well. They usually also retain the "ZR" in the size designation.