Motorcycle Tire Size Conversion Chart: Metric, Inch and Equivalent Sizes
Learn how to read motorcycle tire sizes in metric and inch systems, convert between them, and find equivalent tire sizes for proper fitment. Includes charts and explanation of width, aspect ratio, rim diameter, and overall diameter.
Contents
- What Do the Numbers on a Motorcycle Tire Mean?
- Motorcycle Tire Size Conversion Chart: Metric to Inches
- Motorcycle Off-Road Tire Size Conversion Chart
- Motorcycle Alphanumeric Tire Sizes to Metric
- How to Convert Motorcycle Tire Sizes Metric to Inches
- Aspect Ratio, Sidewall Height, and Overall Diameter
- Equivalent Tire Sizes and Replacement Fitment
What Do the Numbers on a Motorcycle Tire Mean?
Motorcycle tires use two main sizing systems: metric (e.g., 120/70-17) and inch (e.g., 2.75-21). The numbers on the sidewall indicate the tire's dimensions, construction, and rim diameter. Understanding them is essential for choosing the right replacement tire.
In metric sizes, the first number is the section width in millimeters. The second number is the aspect ratio, the sidewall height as a percentage of the width. The letter after the aspect ratio indicates construction (R for radial, B for bias-belted, or no letter for bias-ply). The last number is the rim diameter in inches.
In inch sizes, such as 2.75-21, the first number is the section width in inches (2.75 in), and the second number is the rim diameter in inches (21 in). A tire marked 3.00-18 would be 3 inches wide and fit an 18-inch rim.
Some older bikes use alphanumeric tire sizes like MH90-21. These are being phased out in favor of metric markings, but conversion charts help match them to modern sizes.
Motorcycle Tire Size Conversion Chart: Metric to Inches
The table below shows common metric tire sizes and their approximate inch equivalents. Note that these are nominal equivalents, not exact matches, because aspect ratio affects overall diameter.
| Metric Size | Inch Equivalent (approx.) | Rim Diameter |
|---|---|---|
| 80/90-17 | 2.75-17 | 17 in |
| 90/90-18 | 3.00-18 | 18 in |
| 100/90-18 | 3.25-18 / 3.50-18 | 18 in |
| 110/90-18 | 4.00-18 | 18 in |
| 120/80-18 | 4.50-18 | 18 in |
| 120/90-18 | 4.50-18 / 5.10-18 | 18 in |
| 130/80-17 | 5.00-17 | 17 in |
| 140/80-17 | 5.50-17 | 17 in |
| 80/100-21 | 2.75-21 | 21 in |
| 90/90-21 | 3.00-21 | 21 in |
For example, a 120/70-17 tire has a section width of 120 mm (4.72 in), an aspect ratio of 70, and a 17-inch rim. Its sidewall height is 70% of 120 mm = 84 mm (3.31 in). The overall diameter is 17 in + 2 × 3.31 in = 23.62 in. A equivalent inch tire might be 4.50-17, but check the actual overall diameter before swapping.
Motorcycle Off-Road Tire Size Conversion Chart
Off-road motorcycles, especially dirt bikes and dual-sports, often use inch-based front tire sizes like 2.75-21 or 90/90-21 metric. The table below covers common off-road conversions.
| Inch Size | Metric Equivalent | Typical Use |
|---|---|---|
| 2.50-21 | 80/90-21 | Small dirt bikes |
| 2.75-21 | 80/100-21 | Enduro front |
| 3.00-21 | 90/90-21 | Common front |
| 3.25-21 | 90/100-21 | Larger front |
| 4.00-18 | 110/100-18 | Rear |
| 4.50-18 | 120/90-18 | Rear |
| 5.10-17 | 130/90-17 | Supermoto rear |
Motorcycle Alphanumeric Tire Sizes to Metric
Alphanumeric tire sizes, such as MH90-21, were common on 1970s and 1980s motorcycles. They combine letters and numbers, where the letter indicates the tire's load/speed rating and profile, and the number is the aspect ratio (90 means 90% aspect ratio). They are being replaced by metric sizes.
| Alphanumeric Size | Metric Equivalent |
|---|---|
| MH90-21 | 80/90-21 |
| MJ90-18 | 90/90-18 |
| ML90-18 | 90/90-18 (wider) |
| MM90-16 | 100/90-16 |
| MR90-18 | 110/90-18 |
How to Convert Motorcycle Tire Sizes Metric to Inches
To convert a metric tire size to inches, follow these steps:
- Convert the section width from millimeters to inches: divide by 25.4.
Example: 120 mm ÷ 25.4 = 4.72 in. - Calculate the sidewall height: multiply the width (in mm) by the aspect ratio (as a decimal), then convert to inches.
Example: 120 mm × 0.70 = 84 mm; 84 mm ÷ 25.4 = 3.31 in. - Calculate the overall diameter: rim diameter + 2 × sidewall height.
Example: 17 in + 2 × 3.31 in = 23.62 in. - Use the width and overall diameter to find a suitable inch-size tire with similar dimensions.
Aspect Ratio, Sidewall Height, and Overall Diameter
The aspect ratio is a percentage that expresses the sidewall height relative to the section width. A lower aspect ratio means a shorter sidewall and usually a wider, sportier tire. For example, a 120/70-17 has a 70% aspect ratio, while a 120/90-17 has a 90% aspect ratio. The 120/70 has a sidewall height of 84 mm, whereas the 120/90 has 108 mm, resulting in a larger overall diameter: 23.62 in vs. 25.43 in. Changing aspect ratio affects speedometer accuracy, ground clearance, and suspension geometry.
Equivalent Tire Sizes and Replacement Fitment
When replacing a tire, it's important to choose a size that fits the rim and has an overall diameter within about 3% of the original to avoid clearance issues and large speedometer error. For example, a 2.75-21 inch tire can be replaced by a 80/100-21 metric tire, as both are about 2.75–3.15 inches wide and have a similar overall diameter on a 21-inch rim. Always refer to the motorcycle manufacturer's recommended sizes and consult a tire professional.
Remember that nominal sizes are not exact; actual dimensions vary between manufacturers and tire models. The conversion chart provides starting points, but final selection should be based on actual measured diameter and rim compatibility.