Tire Size Calculator & Speedometer Check
Compare metric tire sizes to calculate diameter, sidewall height, and speedometer error. Visual overlay for tire fitment.
Old Tire (OEM)
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About
Changing tire sizes affects a vehicle's geometry, clearance, and instrumentation. A taller tire increases ground clearance but causes the speedometer to read lower than actual speed. Conversely, a smaller tire increases engine RPM at highway speeds. This calculator decodes metric tire data (e.g., 205/55R16) into physical dimensions.
Understanding the "rolling diameter" is critical for All-Wheel Drive (AWD) systems, where diameter mismatch can damage differentials. This tool provides the percentage difference to ensure the new setup remains within the manufacturer's safe range (typically 3%).
Formulas
Tire diameter D is the sum of the rim diameter (converted to mm) and two sidewall heights.
Speedometer error ratio R:
Reference Data
| Tire Code | Sidewall (mm) | Diameter (mm) | Circumference (mm) | Rev/km |
|---|---|---|---|---|
| 195/65R15 | 126.75 | 634.5 | 1993 | 501 |
| 205/55R16 | 112.75 | 631.9 | 1985 | 503 |
| 225/45R17 | 101.25 | 634.3 | 1992 | 502 |
| 245/40R18 | 98.00 | 653.2 | 2052 | 487 |
| 255/35R19 | 89.25 | 661.1 | 2076 | 481 |
| 275/30R20 | 82.50 | 673.0 | 2114 | 473 |
| 305/30R21 | 91.50 | 716.3 | 2250 | 444 |
| 315/70R17 | 220.50 | 872.8 | 2742 | 364 |