+2.19%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 265/50R20, fitting 285/45R21 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.9 km/h | 25 mph | 25 mph |
| 60 km/h | 61.3 km/h | 37 mph | 38 mph |
| 80 km/h | 81.7 km/h | 50 mph | 51 mph |
| 100 km/h | 102.2 km/h | 62 mph | 63 mph |
| 120 km/h | 122.6 km/h | 75 mph | 76 mph |
Full comparison
265/50R20
Overall diameter773.0 mm · 30.43 in
Section width265 mm · 10.43 in
Sidewall height132.5 mm · 5.22 in
Circumference2428.5 mm · 95.6 in
Revolutions per mile663
Revolutions per km412
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
+2.19%Safe swap — speedometer error is minor
Original
Overall diameter773 mm · 30.43 in
Sidewall height132.5 mm
Circumference2,428.5 mm
Revolutions / mile662.7
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 +2.19%. 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 | 40.9 km/h | 25 mph | 25 mph |
| 60 km/h | 61.3 km/h | 37 mph | 38 mph |
| 80 km/h | 81.7 km/h | 50 mph | 51 mph |
| 100 km/h | 102.2 km/h | 62 mph | 63 mph |
| 120 km/h | 122.6 km/h | 75 mph | 76 mph |
See individual specs: 265/50R20 · 285/45R21. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.