When it’s time for new tires, most consumers rely on experts in the auto industry to tell them what kind of tires they need. Once they’re installed you go happily along your way. However, what if you were educated about types of tires and what all those little numbers on the sides mean? Welcome to Tires 101!

Tire Sizes

If you don’t know what tire size you need—and they come in a variety of sizes, there are ways to determine the size. From the 15-inch used on most passenger cars to the 20-inch tires used on large trucks—it’s easy to find out the size. First look in the owner’s manual from the manufacturer. It will list the size of tires installed when it was originally built.

Unless you intend to change the wheel (rim) size, if your vehicle has 17-inch tires, stick with that size. Also, auto manufacturers are much more innovative than they used to be. Today’s vehicles have a wide variety of sizes depending on model. That means a passenger vehicles are using larger tires so dig out that manual or check the manufacturer’s website.

Tire Tread Depths

Tires also come in different tread depths. A tire’s depth is measured from the top of the tread to the bottom of the tires deepest tread groove. Tire depth is measured by 32nds of an inch and for some that can be tricky to figure out with a standard ruler. Invest in a tread width gauge to calculate the depth. Depth is important in determining if your tires still have useful life in them. You can also use the “Lincoln Penny” test. Stick a penny in a tread on your tire and if you can see the President’s entire head, it’s time for new tires.

Image via Flickr by vagawi

Image via Flickr by vagawi

Tire Widths

There are also varying tire widths that are measured in millimeters. Depending on the make and model of your vehicle, tire width ranges from 185mm to 335mm. According to Edumnds.com, the width is the “distance from sidewall edge to sidewall edge.” The larger the millimeter size, the larger the tire size will be.

When In Doubt, Know What All Those Numbers Mean

If you really want a tire education, once you learn what all those numbers means on the sides of a tire, you’ll be confident when it’s time to buy new ones. Here’s a breakdown based on a common tire, P205/55R/16 89H M&S:

  • First Letter – Most tires have a number that starts with the letter “P” for “passenger.” Other first letters used are LT for light truck, C for commercial and T for temporary—or what’s printed on a spare tire. This letter is a must know to determine tire sizes.
  • Tire Width – The next three numbers on a tire reveal the tire’s width. Using our example, the tire is 205mm in width.
  • Aspect Ratio – The next number in our example, 55, represents the tire’s aspect ratio—or the height of the sidewall relative to its width. This portrays the tire’s absorption ratio. For example, the higher the number, the better the tires will perform in off-road conditions.
  • Radial – Next is the R, which stands for radial. This means the tire is a radial tire. Radial tires are manufactured in layers and are considered some of the best out there today.
  • Diameter – The number 16 in our example means the diameter of the tire—or 16 inches.
  • Load and Speed Ratings – For those who are really interested in becoming a tire expert, the next series in our example, “89H” stands for load and speed ratings. The 89 shows the load is 1,279 pounds per tire. The H reveals the speed rating. In our example, the H reveals the tire will perform up to 130 mph.
  • Weather – Finally, we see M+S. This stands for mud and snow so you know it’s a good all-weather tire for most road conditions. If a tire lacks the M+S, it’s a summer tire.

That’s a wrap up on Tire 101 and if you want to further educate yourself, head on over to miata.net where they office a tire calculator based on type of vehicle.