Get down to eye level with your tires, and you may notice the hieroglyphic-like sequence of letters and numbers imprinted into each sidewall.
The numbers and letters at the end of the sequence that you see on your sidewall correspond to what’s called the “tire service description” — three or 4 figures that signify both the tire load index and the tire speed rating. The load index indicates how much weight a tire can safely carry at maximum air pressure, and the speed rating represents the maximum operating speed.
If you’re still confused, if you just need to find out your tire speed rating or if you have no idea what the tire load index or Z speed means, then you’re in the right place. We’re here to help you get a grasp on what those mysterious codes on your tires mean, how much weight your tires can carry and the maximum speed capability when the tire is properly inflated.
Tire load index
Want to find your tire load index, also known as the load carrying capacity? Locate the two or 3 digits at the end of your tire size — the fancy, technical name for the long string of letters and numbers encircling your tire — that precede the single letter.
For example, if you see “88S” at the end of the tire size, the load index would be 88. So what does that mean for you? Generally speaking, the higher the tire load index, the more weight the tire can safely carry. In the case of a tire with an 88 load index, that tire would be able to support 1,235 pounds at maximum air pressure. When you multiply that amount for a single tire by four, your vehicle’s maximum load capacity would be 4,940 pounds.
Tire speed rating
To find your tire speed rating to reference the single letter at the end of your tire size — this will be where you’ll find the speed rating for all tires manufactured since 1991.
The most common tire speed ratings are L, M, N, P, Q, R, S, T, U, H, Y and (Y) — letters that designate speeds ranging from 75 to above 186 miles per hour.
Unless you’re regularly driving on — or dreaming about driving on — the Autobahn in Germany, you may have never even heard of Z-speed tires. Sound fast, don’t they? Well, that’s because they are. When they were first created, Z-speed tires were meant to represent the highest tire speed rating ever — speeds that exceeded 149 miles per hour.
What you might not know is that, since Z-speed tires were created, tires with even higher tire speed ratings have been introduced: enter the W-speed, Y-speed, and (Y) speed tires. Tires with speed ratings of W, Y, and (Y) can withstand the highest speeds, such as those you’d find when driving an exotic sports car, of 168 to 186 miles per hour and above.
If you've ever wondered how old your tires, it's an easy bit of information to find out tires actually have a "born-on" date, and it's listed right on the tire itself.
How to find your tire load index and tire speed rating
If you’re unable to clearly read the tire size from the sidewall of your tire, you can also locate your tire load index and tire speed rating in your owner’s manual, inside your glove compartment and gas tank hatch and on your driver’s side doorjamb. Of course a tire professional will always be willing to help.