Deciphering Your Tire — A Method to the Madness

Have you ever looked at a tire and wondered what all of the numbers and letters listed on the sidewall mean? Have you ever thought that all those letters and numbers look kind of random and meaningless? Of course you have!

At first glance, you would be forgiven for thinking that tire information looks like something that a team of drunken monkeys banging away at typewriters would crank out. However, after a little bit of time and effort you will be able to decode all of the wonderful information on your tire yourself … amazing your friends, impressing your co-workers, and mystifying onlookers.

For our examples, the tire that we will be referring to is the rear tire from a 2003 Softail® Deuce® (although the information applies equally well to all tires). The relevant information from the Deuce tire is:

Dunlop K591 160/70 B 17 M/C 73V Max. Load 365 kg (805 lbs) at 290 kpa (42 psi) Cold

Dunlop K591

This series of letters indicates the tire manufacturer and model number. It is important to make sure that your front and rear tires match; that they are the same model made by the same manufacturer.

Tires vary in construction strength, material composition, warm-up requirements, and ride quality. Due to this, most modern tires are designed to be used as a matching set. It is potentially dangerous to ride on mismatched tires, as these tires will behave differently under different road stresses.

160/70
This first set of numbers, the designation 160/70, is a metric description for the size of the tire. 160 is the width of the tire in millimeters. Simple enough, right? The number 70 is a little more complex, however. To understand what it means, you first have to understand how the height of tires is measured.

The height of a tire is not how tall a tire is; instead, the height of a tire is determined by measuring from the inside diameter of the tire (the big hole where the rim fits) to the top of the tire's tread. 70 is the aspect ratio that describes the height of a tire as a ratio of the tire's width. Confused yet? Put simply, the number 70 means that the tire is 70% as tall as it is wide. Thus, if your tire is 160 mm wide, and the number behind the slash is 70, your tire height is 70% of 160, or 112 mm.

What if your tire doesn't have a number that looks like 160/70? What if your tire has something that looks like MT90 instead? This letter designation still tells you the width and height of your tire, just in a slightly different format. In this form, the letter M means that the tire is designed for use on motorcycles. The letter T tells you the tire width code (see table below), which can be used to determine the width of the tire. 90 is the aspect ratio of the height of the tire in terms of the tire's width (see above).

Front Tires Letter Designation Metric Conversion
MH90 80/90
MJ90 90/90
MM90 100/90
MN90 110/90
MR90 120/90
MT90 130/90
Rear Tires Letter Designation Metric Conversion
MN90 110/90
MP85 120/90
MT90 130/90
MU90 140/90
MV85 150/90

Knowing your tire's height and width will tell you a lot about the handling characteristics of a particular tire. For instance, a narrow tire is great for low-speed maneuverability, but its narrow dimensions make it relatively unstable at high speeds. Wider tires are great for high-speed stability, but their width makes them difficult to maneuver in tight spaces (parking lots, city streets, and that pesky driver's license test).

The height of a tire can also tell you a lot about a tire's ride quality. Tires with a high aspect ratio usually translates into a softer, more comfortable ride that is well suited to long-distance hauls. However, because the sidewalls of this type of tire flex a fair amount (this is what gives a tire its cushy feel) they are not well suited to high speed cornering. Tires with a small aspect ratio typically off