Tire Size Calculator & Speedometer Error
Compare stock and aftermarket tire sizes. Visualize diameter differences, calculate speedometer error, and check ground clearance changes.
Stock Tire
New Tire
| Metric | Stock | New |
|---|---|---|
| Diameter (mm) | - | - |
| Sidewall (mm) | - | - |
| Circum. (mm) | - | - |
About
Modifying a vehicle's tire size affects more than just aesthetics. Changing the overall diameter of the wheel assembly alters the effective gear ratio, speedometer reading, and ride height. Manufacturers generally recommend staying within a 3% variance of the stock diameter to avoid confusing the ABS and Traction Control systems.
This calculator performs a side-by-side analysis of "Stock" versus "New" tire specifications. It computes the precise sidewall height and circumference, alerting you if the new setup poses a safety risk or requires a speedometer recalibration.
Formulas
Tire diameter is derived from the standard ISO metric sizing format (e.g., Width/Aspect R-Rim).
Where:
- W = Tire Width in mm
- AR = Aspect Ratio (percentage of width)
- Rim = Wheel Diameter in inches
Speedometer error is calculated by the ratio of circumferences:
Reference Data
| Tire Spec | Diameter (mm) | Diameter (in) | Sidewall (mm) | Circumference (mm) | Revs/km |
|---|---|---|---|---|---|
| 195/65 R15 | 634.5 | 25.0 | 126.8 | 1,993 | 502 |
| 205/55 R16 | 631.9 | 24.9 | 112.8 | 1,985 | 504 |
| 225/45 R17 | 634.3 | 25.0 | 101.3 | 1,993 | 502 |
| 245/40 R18 | 653.2 | 25.7 | 98.0 | 2,052 | 487 |
| 275/35 R19 | 675.1 | 26.6 | 96.3 | 2,121 | 471 |
| 305/30 R20 | 668.0 | 26.3 | 91.5 | 2,098 | 476 |