Quick Summary: How to read motorcycle tire sizes like 90/90-18 51P. Learn section width, aspect ratio, rim diameter, load index and speed rating — plus how to choose the right replacement tire.
| Marking | Meaning |
|---|---|
| 90 | Section width: ~90 mm |
| 90 | Aspect ratio: 90% |
| 18 | Rim diameter: 18 inches |
| 51 | Load index: 195 kg max per tire |
| P | Speed rating: 150 km/h max |
In short: 90/90-18 51P = 90mm width + 90% sidewall + 18-inch rim + 195kg load + 150km/h speed.
The first number is the tire's nominal section width in millimeters. For 90/90-18, that's approximately 90mm at the widest point.
Note: This is a standardized designation, not an exact measurement. Actual width varies slightly by brand, model and rim width.
The second number is the sidewall height as a percentage of section width.
90mm × 90% ≈ 81mm sidewall height
Higher aspect ratio = taller sidewall = more comfort
Lower aspect ratio = shorter sidewall = sportier handling
The tire fits an 18-inch rim. This must match exactly — you cannot mount an 18-inch tire on a 17-inch rim.
Load index is the maximum weight per tire under rated conditions.
Important: 51 = 195kg per tire, not total bike weight. A bike with two 51-index tires does not automatically carry 390kg.
When replacing tires, the new load index must meet or exceed the manufacturer's original spec.
Speed rating is the maximum rated speed under test conditions — not a recommended riding speed.
| Rating | Max Speed |
|---|---|
| J | 100 km/h |
| N | 140 km/h |
| P | 150 km/h |
| S | 180 km/h |
| H | 210 km/h |
| V | 240 km/h |
| System | Format | Example |
|---|---|---|
| Metric | Width/Aspect-Ratio-Rim | 90/90-18 |
| Inch | Width-Rim | 2.75-18 |
Warning: 2.75-18 and 90/90-18 are not automatically interchangeable. Different constructions, measurement standards and approved rim widths affect actual fitment. Always check the manufacturer's spec before substituting.
| Tire Size | Width | Aspect | Rim | Common Use |
|---|---|---|---|---|
| 80/90-17 | 80mm | 90% | 17" | Scooter |
| 90/90-17 | 90mm | 90% | 17" | Underbone / commuter |
| 90/90-18 | 90mm | 90% | 18" | Commuter / standard |
| 100/90-18 | 100mm | 90% | 18" | Cruiser |
| 110/90-17 | 110mm | 90% | 17" | Sport / naked |
| 120/80-17 | 120mm | 80% | 17" | Sport / supersport |
Check the original tire marking — record the full spec, not just width
Match rim diameter — 17" rim needs 17" tire
Check clearance — fender, swingarm, suspension, chain/belt
Match load index — must meet or exceed factory spec (critical for two-up riding, cargo, delivery)
Match speed rating — must meet or exceed factory spec
| Type | Description | Common Use |
|---|---|---|
| Tube-Type | Requires inner tube | Spoke wheels, older models |
| Tubeless | No inner tube needed | Alloy wheels, modern bikes |
| Bias-Ply | Traditional nylon construction | Commuter, utility, classic bikes |
| Radial | Steel belted construction | High-speed, performance bikes |
Neither construction is universally better — match the tire to your motorcycle's design and intended use.
Not necessarily. They may be used in similar applications but are not automatically interchangeable. Always check the manufacturer's approved size and rim width.
Possibly, but you must verify rim width, fender clearance, swingarm clearance and overall diameter first. A tire that physically fits is not necessarily the correct tire.
51 = load index 195kg per tire. P = speed rating 150 km/h.
For 90/90-18 51P:
90 = section width
90 = aspect ratio
18 = rim diameter
51 = load index
P = speed rating
The golden rule: Never choose a tire based only on width or appearance. Verify the full spec, rim compatibility, load index, speed rating and construction before buying.
Looking for a reliable motorcycle tire manufacturer? Zihai Rubber produces 90/90-17, 90/90-18, 100/90-18, 3.00-17, 3.00-18 and more sizes for scooter, commuter and off-road applications. Contact us for wholesale pricing and OEM options.
