By Justin Mastine-Frost for The Workbench Life
Gone are the days when you could walk into a tire shop, ask for a set of all-season tires, and be presented with one or two options. Now it seems every manufacturer has an endless array of products to choose from, making shopping for tires surprisingly complicated. We thought it would be a good idea to walk you through what all those little numbers and letters on your tires really mean, and how to know you’re getting what you need.
The best place to look for your correct tire size is either in your owner’s manual or on your current tires. Tire size is written out in a slightly bizarre format that looks like this: 205/55R16. (There will be a few numbers beyond that which I will explain later.) Occasionally, you will see either a P or LT in front of the size, which means it’s meant for either a passenger car or a light truck.
It’s always a good idea to use the tire size specified by the manufacturer of your vehicle. If you use a larger or smaller tire, it might rub on the inside of the car’s fenders, cause problems with the car’s traction control systems, or mess with the speedometer. (Speed is measured by wheel sensors which make a calculation based on the original manufacturer’s tire size.)
Load index and speed rating
The load index and speed rating of your tires can be found in the three digits following tire size, and consist of two numbers and a letter. I won’t spend much time on the load index, other than to suggest you stick with the same number or higher. And no, it won’t affect your car’s handling.
Speed rating is where things get interesting. This letter tells you how fast your tire was engineered to travel. Z-rated tires are capable of traveling over 149 mph, and used to be the fastest. Z-rated tires have a different numbering system, where the letter appears ahead of the R in a tire size (315/40ZR19). Y and W tires now go even faster, but are meant for high-end supercars. Z-rated tires are for performance and have cool tread patterns on them, but unless you plan on driving more than 149 mph on a regular basis, spending that extra money is like wearing Italian dress shoes to a construction site.
The three Ts: traction, temperature and tread wear
When it comes to tire shopping, these three factors should be your top consideration. The UTQG, or Uniform Tire Quality Grade, requires manufacturers to display performance ratings on the sidewalls of their tires. Traction rates range from AA to C, and designate the amount of G-forces a tire can sustain in a standardized test. Temperature ratings, from A to C, denote a tire’s ability to dissipate heat, and thus perform at higher speeds. Tread wear ratings start at 100 and increase incrementally, indicating how many times a tire would be able to run through a standardized test loop.
Looking at these three ratings, there are a few things to consider. A high tread wear grade (one over 600) is great if you’re seeking durability, but will mean that the tire is made from a harder rubber, and will probably be noisier on the road. A traction rating of AA, on the other hand, means the tire will deliver good traction, but it’s made from a softer compound, so you’ll be replacing it more than a B- or C-rated tire.
There isn’t a magic answer to it all. Consider your driving habits and performance requirements before you go shopping. Once you know your base criteria, finding the right tire for the job will be a breeze.
Photo: Fred Albert
Mastine-Frost is a factory-trained BMW technician, automotive
journalist from Vancouver, B.C., and frequent contributor to The