Once up on time, understanding the size of a motorcycle tire was simple. If you bought a 4.00 x 18 tire, it was four inches wide and fit an 18-inch rim. All you had to know was what width tires your rim and swingarm would accommodate. As anyone shopping for tires knows, those days are long gone. Modern tire shoppers face a dizzying array of alphanumeric codes that are anything but intuitive. However, they do offer more information. To access that info, you need a translator for reading motorcycle tires, and that’s where this article comes in.
Let’s understand the codes, using a Michelin Power 6 rear tire as an example in a common size: 190/55 ZR 17 M/C (73W).
Yes, that’s a lot of numbers, letters, punctuation marks, and spaces. It would make for a great password, except that there are no lowercase letters. Fortunately, if we break it down, it starts to make sense.
Let’s start with the 190/55. You won’t like this if you’re metric- and arithmetic-phobic, as both are involved. The 190 tells you the tire is 190mm wide. In British Imperial units, which the British use inconsistently, that means 7.5 inches wide. Okay, we have gotten the metric part out of the way.
The 55 is a percentage—yep, math has entered the chat, albeit elementary math. With the tire’s width, you get what is called the aspect ratio—aka the tire’s height, which is also referred to as profile.
As it is a percentage rather than a specific measurement, it depends on the tire’s width to determine its height. It tells you that the tire’s height is 55 percent of its width. In the case of the 190/55 ZR 17 M/C (75W) Michelin tire, that means it is 104.5mm high (190mm x .55). If you don’t like millimeters, that’s 4.1 inches.
So, the first number after the slash is a percentage based on the first number. This will help you compare two tires that are the same width. In that case, a larger second number indicates a higher profile tire, while a smaller second number reflects a lower profile tire. Typically, a higher profile (larger number) means more comfort, and a lower profile (smaller number) suggests better handling in corners due to less tire deformation.
If you’re going to compare different tire widths, prepare to do a bit of math. For instance, in the dirt bike world, some 21-inch front tires are 90/90, while others are 80/100. In the case of the 90/90, the tire’s height is 81mm (90mm x .9). The 80/100 has a height of 80mm (80 x 1.0). So, one tire has 90 as the second number, and another has 100 as the second number, yet the two have virtually the same height.
Okay, with the height and width out of the way, we move on to the first letters in 190/55 ZR 17 M/C (75W). The Z tells you it’s good for 149 mph. The R tells you it is a radial tire. If the letter is a B, it’s a bias-ply tire. Sometimes, these letters are left out, so you’ll have to ask if it’s a radial or bias-ply tire. If there’s no Z, sit tight—an additional speed rating comes later.
After the ZR, R, or B, the number is the rim size the tire will fit on. This is where things go a bit sideways. While the tire’s width is in metric millimeters, the rim size is measured in inches—you know, the Exchequer Standards of 1826 (aka British Imperial units). If you must know the metric equivalent of a 17-inch rim, it’s 431.8mm.
Next up is M/C. We’re pretty sure our readers can figure out that that tells you it’s a motorcycle tire. See? Reading motorcycle tires isn’t always difficult.
Now we’re onto the truly arcane alphanumeric portion of the tire. Unless you work in a tire shop and are good at memorization, you will need a chart to understand these numbers and letters. The numbers and letters are arbitrary.
The number, 75 in this example, tells you the load index—how much weight the tire can handle. Roughly half the weight of the bike (plus riders and cargo) is on each tire. So, that tire has to be able to accept that weight, or it may fail. The index numbers range from 20 (176 pounds) to 99 (1709 pounds.
In the case of the 75 on the Michelin Power 6 in question, the tire can handle 853 pounds. If the total weight of your motorcycle, riders, and cargo is less than 1700 pounds in everyday riding, which is any bike that uses the Power 6, you’re good to go. Keep in mind that the higher weight rating will also mean a more stable tire under heavy braking, where the load on the front wheel can be very high.
Here’s a weight index rating chart from Michelin:
Finally, we get to that lonely, enigmatic letter at the end. Its job is to tell you the maximum speed the tire can handle. Speed ratings range from 31 mph for a B-rated tire to 186 mph for a Y-rated tire. The Power 6 is a W-rated tire, so 168 mph is the maximum. However, because the W is in parentheses—(73W)—Michelin allows speeds of over 168 mph without specifying a specific number. The next level up is Y, which is 186 mph, so you would want to keep a (W) tire under that. Feel free to take the Michelin Power 6 to the track; we did when we tested it.
Most tires are between P and W. You would probably guess that the maximum allowable speed increases as you move further into the alphabet. You would be right, with one exception. H is dropped in between U and V. Does that make sense? Of course not. However, we have another chart from Michelin for your convenience:
When you have the tire in your hands, you can also check the DOT code that tells you when the tire was made. Again, the code is a bit tricky. The first two numbers are the week that a tire was made. The second two numbers are the year. So, a tire made the week that this story was written would have 4224 on it, as this is the 42nd week of 2024. You don’t want to run a tire over six years old, so make sure the manufacturing date is at least six years from when you expect to replace the tire.
Once the tires are on your bike, we also have essential info on proper air pressure for your tire. Getting that wrong can result in poor handling and, in the worst case, tire failure.
Now, when it comes time to buy a set of tires for your motorcycle, none of those numbers on the side of the tire will be a mystery. You now know all about reading motorcycle tires.
