-7.07%Not recommended — recalibrate the speedometer
Speedometer error
With a speedometer calibrated to 285/60R20, fitting 285/45R21 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 37.2 km/h | 25 mph | 23 mph |
| 60 km/h | 55.8 km/h | 37 mph | 35 mph |
| 80 km/h | 74.3 km/h | 50 mph | 46 mph |
| 100 km/h | 92.9 km/h | 62 mph | 58 mph |
| 120 km/h | 111.5 km/h | 75 mph | 69 mph |
Full comparison
285/60R20
Overall diameter850.0 mm · 33.46 in
Section width285 mm · 11.22 in
Sidewall height171.0 mm · 6.73 in
Circumference2670.4 mm · 105.1 in
Revolutions per mile603
Revolutions per km374
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
-7.07%Not recommended — recalibrate the speedometer
Original
Overall diameter850 mm · 33.46 in
Sidewall height171 mm
Circumference2,670.4 mm
Revolutions / mile602.7
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 -7.07%. 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 | 37.2 km/h | 25 mph | 23 mph |
| 60 km/h | 55.8 km/h | 37 mph | 35 mph |
| 80 km/h | 74.3 km/h | 50 mph | 46 mph |
| 100 km/h | 92.9 km/h | 62 mph | 58 mph |
| 120 km/h | 111.5 km/h | 75 mph | 69 mph |
See individual specs: 285/60R20 · 285/45R21. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.