Motorcycle Tire Sizes Guide

Width

The first number or second letter in a tire size represents the nominal width. Width is measured in a straight line from the furthest point on one sidewall, across the tread, to the furthest point on the opposite sidewall. If there is any question whether or not a larger than OEM tire will fit your bike, you're encouraged to call Tech Service. The different size numbering systems specify widths in different measurements. You can view a width cross reference table below .

Aspect Ratio

You use the aspect ratio to indicate a tire’s cross-sectional profile. A smaller number means a lower profile, and the height-to-width ratio is shown as a percent. For example, if the aspect ratio is 90, that means that your tire’s cross-sectional height is 90% of its width. If the tire has the metric, alpha or low-profile inch numbering system, the aspect ratio will appear immediately after the width.

Motorcycle Speed Rating Chart

A speed rating provides the maximum speed at which a tire can be used based on maximum load and inflation pressure. To find the maximum load and inflation pressure, you’ll need to check the sidewall of the tire. A speed rating also has a letter code, and that is immediately after the width and aspect ratio. It’s part of the three-digit load/speed index. This is on the tire, immediately after the complete size designation. A Z-rated tire has no maximum speed — this rating is for more than 149mph. Check out this table to see what letters correspond with what speed ratings:

Tire Construction

After the speed rating is the tire construction, which has two options: Belted (B) or Radial (R). “Belted” means fiberglass, Kevlar or aramid fiber belts, which add strength and load capacity. But, if a tire does not have (R), then it is bias-ply, which means that it has multiple, overlapping rubber plies. These overlapping plies form a thick layer that is sensitive to overheating and is less flexible.

Rim Diameter

Rim diameter is the diameter of the rim/wheel on which the tire will be mounted, in inches.

Other factors for identifying motorcycle tire sizes

WW: White wall tire

TT: Tube-type tire (requires inner air tube)

TL: Tubeless tire

M/C: Motorcycle tire (in other words, for motorcycles only)

Numbering systems used in motorcycle tire sizing

Let’s check out some examples of how to read motorcycle tire dimensions:

180/55ZR-17

• 180 = width (millimeters)
• 55 = aspect ratio (cross-section height is 55% of the width)
• Z = speed rating (149mph or more)
• R = radial construction
• 17 = diameter (inches)

Metric

Metric tires are most commonly used and can work for sport bikes, cruisers, touring bikes and others. If there is no speed rating, the “M” will precede the metric size. After the speed rating, there will be a “B” or “R” to indicate if it is belted or radial tire construction

MT90S-16

• M = motorcycle tire
• T = width
• 90 = aspect ratio (cross-section height is 90% of width)
• S = speed rating (112mph)
• 16 = wheel diameter (inches)
• [no construction designation after speed rating means that it is bias-ply]

Alpha

The Alpha size designation is most often used for touring motorcycle tires. Even if a tire doesn’t have a“B” designation after the speed rating, it could be belted.

3.25H-19

• 3.25 = width
• H = speed rating
• 19 = rim diameter (inches)
• [aspect ratio not included, but is 100% of the width]

Standard Inch

Often, older motorcycle tires have a Standard Inch System. These tires do not have an aspect ratio designation marking. The cross-section tire height is 100% of the width. An inch-sized tire is bias-ply construction and is not belted. However, these are being phased out and are not used on newer motorcycles.

4.25/85H-18

• 4.25 = width
• 85 = Aspect Ration (Aspect Ration is 85% of the width)
• H = Speed Rating
• 18 = Rim Diameter (inches)

Low Profile Inch

Series Size System when the aspect ratio is unspecified, so it will always be 82% unless it is specifically noted to have an 85% aspect ratio. However, manufacturers are no longer using tires with this size system, so you need to determine the proper ratio with the manufacturer.