Reading Motorcycle Tire Size and Sidewall Markings

The short answer

A motorcycle tire size such as 120/70 ZR17 M/C (58W) contains three kinds of information. The dimensions are 120 (nominal section width in millimeters), 70 (aspect ratio, meaning sidewall height as a percentage of width) and 17 (wheel bead-seat diameter in inches). The construction is the letter before the rim diameter, here R for radial. The service description in parentheses is 58W: a load index (58) and a speed symbol (W). M/C marks the tire as a motorcycle tire, and a Z in the size indicates a high-speed category.

The dimensions are nominal. They describe the tire's designated size, not a guaranteed measurement on your wheel. Dunlop notes that actual mounted width can differ from the stated section width.

Worked example: 120/70 ZR17 M/C (58W)

Marking	Category	Meaning
120	Dimension	Nominal section width, 120 mm
70	Dimension	Aspect ratio: section height is 70% of width, so 120 × 0.70 = 84 mm nominal height
Z	Speed category	High-speed category above 240 km/h (about 149 mph), per Dunlop
R	Construction	Radial
17	Dimension	Wheel bead-seat diameter, 17 inches
M/C	Designation	Motorcycle tire
58	Service description	Load index; Dunlop's table maps 58 to 236 kg (about 520 lb)
W	Service description	Speed symbol

In this format, Z and R appear together as ZR. They answer different questions. Z describes the speed category, and R describes how the tire carcass is built.

Dimensions: width, aspect ratio and rim diameter

The first number is section width in millimeters. The number after the slash is a ratio, not a measurement. To get nominal sidewall height, multiply the width by the ratio expressed as a decimal. Michelin identifies the final number in the size as the wheel's bead-seat diameter in inches, which is the diameter where the tire bead sits on the rim.

The mixed units are normal: millimeters for width and height, inches for rim diameter. To compare nominal overall diameter, convert the rim size to millimeters (1 inch = 25.4 mm) and add the sidewall height twice. For 120/70 ZR17, that is 17 × 25.4 = 431.8 mm, plus 2 × 84 mm, for about 600 mm. This is arithmetic on nominal figures. A specific tire model, inflated on a specific rim, can measure differently.

Construction: R, B or a dash

The character directly before the rim diameter identifies carcass construction. Michelin's examples use:

• R: radial construction (for example, 180/55 ZR17)
• A dash (-): bias construction (for example, 130/90-16)
• B: bias-belted construction (for example, 130/90 B16)

The construction letter identifies the tire type. On its own, it does not tell you whether two constructions can be combined on one motorcycle. That question belongs to the motorcycle and tire manufacturers' documentation.

Service description: load index and speed symbol

The service description is the code in parentheses after M/C. The number is a load index, which refers to a load capacity in the manufacturer's load-index table. It is not a weight in pounds or kilograms. The letter is a speed symbol, which refers to a speed capability.

Example 2: 180/55 ZR17 M/C (73W), a typical rear size. The nominal width is 180 mm and the nominal section height is 180 × 0.55 = 99 mm. The construction is radial, and the bead-seat diameter is 17 inches. Michelin gives load index 73 as 365 kg (about 805 lb). Michelin explains the parenthesized W as a speed capability above 270 km/h (about 168 mph).

Copy the parentheses whenever you write down a service description. The brackets are part of the marking, and dropping them changes what you transcribed.

Other size formats you may see

Not every motorcycle tire uses the metric width/ratio format. Some use inch-based sizes, and some use alphanumeric sizes. Michelin publishes comparison tables between these systems. One example lists the alphanumeric MT90 alongside the metric 130/90.

Use these tables to translate between notations. A table entry shows that two sizes are listed as comparable. It does not approve either size for your motorcycle.

Additional letters and brand-specific codes

Sidewalls often carry more letters and abbreviations than the size and service description. Look up each one in the documentation for the exact tire model. Don't guess a code's meaning from how it looks or from a similar code on another brand. The same applies to the original-equipment specifications for your motorcycle.

Riders searching for a Honda or Harley-Davidson tire markings guide should use the owner's documentation for their model, model year and market. That is the source for the original front and rear specifications. The tire maker's documentation then explains what the tire's own markings mean.

A printable decoding worksheet

If you wanted a PDF guide, the worksheet below does the same job. Copy it into a document or print it. Fill in one sheet per tire position.

MOTORCYCLE TIRE MARKING WORKSHEET

Motorcycle make / model / model year: ____________________
Market (if known):                    ____________________
Position:  [ ] Front   [ ] Rear

Complete marking as printed:          ____________________

Field                     Value      Source document / page
------------------------  ---------  ----------------------
Nominal section width     ________   ______________________
Aspect ratio              ________   ______________________
Nominal height (W x AR)   ________   (calculated)
Construction (R / B / -)  ________   ______________________
Bead-seat diameter (in)   ________   ______________________
Motorcycle designation    ________   ______________________
Load index                ________   ______________________
Speed symbol (with parens)________   ______________________
Additional markings       ________   ______________________

Original specification (motorcycle documentation): __________
Tire model documentation consulted:                __________

How to verify what you decoded

• 1. Record the motorcycle's model, model year and whether the tire is front or rear.
• 2. Transcribe the original tire specifications from the motorcycle's documentation.
• 3. Copy the full sidewall marking exactly, including slashes, letters and parentheses.
• 4. Split it into dimensions, construction and service description using the worksheet.
• 5. Check each interpretation, and any extra letters, against the exact tire model's documentation. Note the document and page for each.
• 6. Compare the decoded marking with the original specification field by field.

This process identifies what a tire is. Whether a different size or construction is acceptable on your motorcycle is a fitment decision. Make that decision with the motorcycle and tire manufacturers' guidance, not from the markings alone.