-2.29%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 275/45R21, fitting 255/50R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.1 km/h | 25 mph | 24 mph |
| 60 km/h | 58.6 km/h | 37 mph | 36 mph |
| 80 km/h | 78.2 km/h | 50 mph | 49 mph |
| 100 km/h | 97.7 km/h | 62 mph | 61 mph |
| 120 km/h | 117.2 km/h | 75 mph | 73 mph |
Full comparison
275/45R21
Overall diameter780.9 mm · 30.74 in
Section width275 mm · 10.83 in
Sidewall height123.8 mm · 4.87 in
Circumference2453.3 mm · 96.6 in
Revolutions per mile656
Revolutions per km408
255/50R20
Overall diameter763.0 mm · 30.04 in
Section width255 mm · 10.04 in
Sidewall height127.5 mm · 5.02 in
Circumference2397.0 mm · 94.4 in
Revolutions per mile671
Revolutions per km417
-2.29%Safe swap — speedometer error is minor
Original
Overall diameter780.9 mm · 30.74 in
Sidewall height123.8 mm
Circumference2,453.3 mm
Revolutions / mile656
New
Overall diameter763 mm · 30.04 in
Sidewall height127.5 mm
Circumference2,397 mm
Revolutions / mile671.4
Speedometer & odometer error
The new tire changes overall diameter by -2.29%. 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 | 39.1 km/h | 25 mph | 24 mph |
| 60 km/h | 58.6 km/h | 37 mph | 36 mph |
| 80 km/h | 78.2 km/h | 50 mph | 49 mph |
| 100 km/h | 97.7 km/h | 62 mph | 61 mph |
| 120 km/h | 117.2 km/h | 75 mph | 73 mph |
See individual specs: 275/45R21 · 255/50R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.