Have you ever wondered what all those markings on your tires actually mean? There are a lot of letters, numbers and symbols on the sidewalls of your tires that translate to a lot of useful information. You’ll find it’s useful to be able to identify a few of these strange hieroglyphics when you’re selecting the proper tires for your motorcycle. Among other things, these markings indicate the size, construction, speed rating and load index of the tire.  


There are three different designations used by manufacturers to indicate tire size: metric, the alpha numeric and inch. Most manufacturers prefer the metric and alpha numeric systems because they provide the most information. As you’ll see below, each system addresses critical information such as width, aspect ratio and rim size.

Metric System

The first two or three digits to the left side of the slash indicate the width of the tire in millimeters. The two digits to the right of the slash indicate the aspect ratio. This number represents the distance between the bead of the tire and the tread. It is a percentage, so if you have a 130/90, the aspect ratio is 90 percent of its width (130mm).  The last two digits indicate the rim size.

Alpha Numeric System

This system is similar to the metric system. However, instead of using numbers to indicate the width of the tire, this system uses letters. Also, the aspect ratio sees the tires as being 85 or 90 percent as wide as they are tall. The last two digits indicate the rim size.

Inch System

The Inch system is not as informative as the preceding methods of measurement because it only represents the width of the tire and the rim size.  

Use the conversion chart below to compare the size relationship between the above designations:


Most manufacturers will use either a “B” for bias-ply or “R” for radial construction.

In short, bias-ply tires have plies running diagonally from one bead to the other, with alternating plies angled in opposite directions. The angle of the plies determines the tire’s strength and flexibility. Radial tires simply have the cords wrapped in a radial pattern around the tire from one shoulder to the other. These tires are more suitable for lightweight, high-performance applications.

Here’s one reason why you need to understand these markings: Due to differences in construction, you must never run a radial front tire with a bias-ply rear tire. Some late model Harley-Davidsons are coming out of the factory with bias-ply front and radial rear tires. This is permissible. Again — don’t run radial front tires with bias-ply rear tires.  

Speed Rating

This letter indicates the maximum speed limit that a tire is designed to handle. The chart below translates what each letter stands for in miles per hour.

Load Index

This number indicates the maximum load that a tire can carry. You must take the weight of the bike, yourself, your passenger and any other additional equipment into consideration when determining the load capacity you will require:

There you have it.  These are the basics and you should find this information to be helpful next time you’re shopping for new rubber.