+2.61%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 285/45R21, fitting 275/55R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 41.0 km/h | 25 mph | 26 mph |
| 60 km/h | 61.6 km/h | 37 mph | 38 mph |
| 80 km/h | 82.1 km/h | 50 mph | 51 mph |
| 100 km/h | 102.6 km/h | 62 mph | 64 mph |
| 120 km/h | 123.1 km/h | 75 mph | 77 mph |
Full comparison
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
275/55R20
Overall diameter810.5 mm · 31.91 in
Section width275 mm · 10.83 in
Sidewall height151.3 mm · 5.95 in
Circumference2546.3 mm · 100.2 in
Revolutions per mile632
Revolutions per km393
+2.61%Safe swap — speedometer error is minor
Original
Overall diameter789.9 mm · 31.1 in
Sidewall height128.3 mm
Circumference2,481.5 mm
Revolutions / mile648.5
New
Overall diameter810.5 mm · 31.91 in
Sidewall height151.3 mm
Circumference2,546.3 mm
Revolutions / mile632
Speedometer & odometer error
The new tire changes overall diameter by +2.61%. 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 | 41 km/h | 25 mph | 26 mph |
| 60 km/h | 61.6 km/h | 37 mph | 38 mph |
| 80 km/h | 82.1 km/h | 50 mph | 51 mph |
| 100 km/h | 102.6 km/h | 62 mph | 64 mph |
| 120 km/h | 123.1 km/h | 75 mph | 77 mph |
See individual specs: 285/45R21 · 275/55R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.