If you look at the sidewall of your tires, you will see a string of letters and numbers that tell you the tire size. On most cars, there is another letter directly following the tire size, which tells you the speed rating of the tire. These letters let you know how fast you can safely drive on the tires. It's not necessarily all that important in America, where it is illegal to drive over 70 miles per hour in most states, but in countries such as Germany, with its famous autobahn and the Isle of Man where many roads don't have speed limits, it is important to have tires that can hold up under high-speed conditions.
The lowest-rated tires you can buy are M rated. They are approved for usage up to 81 miles per hour. M-rated tires are used on passenger cars, but only as temporary spares. The next step up from an M-rated tire is an N-rated tire. N-rated tires are approved for speeds up to 87 miles per hour. After that are P-rated tires (up to 93 mph), Q-rated tires (up to 99 mph) and R-rated tires (up to 106 mph). N-rated through R-rated tires are only available for light trucks and not passenger vehicles.
The next step up in speed ratings is an S-rated tire, which is approved up to 112 mph. After that are T-rated tires (up to 118 mph), U-rated tires (up to 124 mph) and H-rated tires (up to 130 mph). All of these tires, from S through H, can be found for both passenger cars and light trucks. H-rated tires are the most common and are found standard on most passenger cars in America.
After H-rated tires come V-rated tires. V-rated tires are approved for use up to 149 miles per hour. Z-rated tires are approved for speeds in excess of 149 miles per hour. These tires can only be used on passenger cars.
Often times, you will find Z-rated tires for sale at a sharp discount from their lower-rated counterparts. This is because Z-rated tires wear out much faster than the lower-rated tires. While it might be tempting to save some money up front, it's unlikely you'll be able to drive over 150 miles per hour in America, and you'll end up paying for it in the long run when you have to purchase new tires much sooner than you would if you bought lower-rated ones.
Z-rated tires used to be the highest-rated tires available, but with more modern cars being able to reach speeds way above 149 miles per hour, two new speed ratings have been introduced. W-rated tires have been tested safe at speeds up to 168 miles per hour, and Y-rated tires have been proven safe at speeds up to 186 miles per hour. But unless you're driving a supercar on the autobahn, it's unlikely you'll ever need tires such as these.
