-3.85%Borderline — noticeable speedometer/odometer error
Speedometer error
With a speedometer calibrated to 285/55R20, fitting 285/45R21 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 38.5 km/h | 25 mph | 24 mph |
| 60 km/h | 57.7 km/h | 37 mph | 36 mph |
| 80 km/h | 76.9 km/h | 50 mph | 48 mph |
| 100 km/h | 96.2 km/h | 62 mph | 60 mph |
| 120 km/h | 115.4 km/h | 75 mph | 72 mph |
Full comparison
285/55R20
Overall diameter821.5 mm · 32.34 in
Section width285 mm · 11.22 in
Sidewall height156.8 mm · 6.17 in
Circumference2580.8 mm · 101.6 in
Revolutions per mile624
Revolutions per km387
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
-3.85%Borderline — noticeable speedometer/odometer error
Original
Overall diameter821.5 mm · 32.34 in
Sidewall height156.8 mm
Circumference2,580.8 mm
Revolutions / mile623.6
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 -3.85%. 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 | 38.5 km/h | 25 mph | 24 mph |
| 60 km/h | 57.7 km/h | 37 mph | 36 mph |
| 80 km/h | 76.9 km/h | 50 mph | 48 mph |
| 100 km/h | 96.2 km/h | 62 mph | 60 mph |
| 120 km/h | 115.4 km/h | 75 mph | 72 mph |
See individual specs: 285/55R20 · 285/45R21. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.