-0.39%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 285/50R20, fitting 285/45R21 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.8 km/h | 25 mph | 25 mph |
| 60 km/h | 59.8 km/h | 37 mph | 37 mph |
| 80 km/h | 79.7 km/h | 50 mph | 50 mph |
| 100 km/h | 99.6 km/h | 62 mph | 62 mph |
| 120 km/h | 119.5 km/h | 75 mph | 74 mph |
Full comparison
285/50R20
Overall diameter793.0 mm · 31.22 in
Section width285 mm · 11.22 in
Sidewall height142.5 mm · 5.61 in
Circumference2491.3 mm · 98.1 in
Revolutions per mile646
Revolutions per km401
285/45R21
Overall diameter789.9 mm · 31.10 in
Section width285 mm · 11.22 in
Sidewall height128.3 mm · 5.05 in
Circumference2481.5 mm · 97.7 in
Revolutions per mile649
Revolutions per km403
-0.39%Safe swap — speedometer error is minor
Original
Overall diameter793 mm · 31.22 in
Sidewall height142.5 mm
Circumference2,491.3 mm
Revolutions / mile646
New
Overall diameter789.9 mm · 31.1 in
Sidewall height128.3 mm
Circumference2,481.5 mm
Revolutions / mile648.5
Speedometer & odometer error
The new tire changes overall diameter by -0.39%. With a speedometer calibrated to the original tire, that same percentage is how far the speedometer and odometer read off — a taller tire means your true speed is higher than indicated and the odometer under-counts distance.
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.8 km/h | 25 mph | 25 mph |
| 60 km/h | 59.8 km/h | 37 mph | 37 mph |
| 80 km/h | 79.7 km/h | 50 mph | 50 mph |
| 100 km/h | 99.6 km/h | 62 mph | 62 mph |
| 120 km/h | 119.5 km/h | 75 mph | 74 mph |
See individual specs: 285/50R20 · 285/45R21. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.