-6.35%Not recommended — recalibrate the speedometer
Speedometer error
With a speedometer calibrated to 305/55R20, fitting 285/45R21 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 37.5 km/h | 25 mph | 23 mph |
| 60 km/h | 56.2 km/h | 37 mph | 35 mph |
| 80 km/h | 74.9 km/h | 50 mph | 47 mph |
| 100 km/h | 93.6 km/h | 62 mph | 58 mph |
| 120 km/h | 112.4 km/h | 75 mph | 70 mph |
Full comparison
305/55R20
Overall diameter843.5 mm · 33.21 in
Section width305 mm · 12.01 in
Sidewall height167.8 mm · 6.60 in
Circumference2649.9 mm · 104.3 in
Revolutions per mile607
Revolutions per km377
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
-6.35%Not recommended — recalibrate the speedometer
Original
Overall diameter843.5 mm · 33.21 in
Sidewall height167.8 mm
Circumference2,649.9 mm
Revolutions / mile607.3
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 -6.35%. 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 | 37.5 km/h | 25 mph | 23 mph |
| 60 km/h | 56.2 km/h | 37 mph | 35 mph |
| 80 km/h | 74.9 km/h | 50 mph | 47 mph |
| 100 km/h | 93.6 km/h | 62 mph | 58 mph |
| 120 km/h | 112.4 km/h | 75 mph | 70 mph |
See individual specs: 305/55R20 · 285/45R21. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.