+0.99%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 275/45R20, fitting 255/50R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.4 km/h | 25 mph | 25 mph |
| 60 km/h | 60.6 km/h | 37 mph | 38 mph |
| 80 km/h | 80.8 km/h | 50 mph | 50 mph |
| 100 km/h | 101.0 km/h | 62 mph | 63 mph |
| 120 km/h | 121.2 km/h | 75 mph | 75 mph |
Full comparison
275/45R20
Overall diameter755.5 mm · 29.74 in
Section width275 mm · 10.83 in
Sidewall height123.8 mm · 4.87 in
Circumference2373.5 mm · 93.4 in
Revolutions per mile678
Revolutions per km421
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
+0.99%Safe swap — speedometer error is minor
Original
Overall diameter755.5 mm · 29.74 in
Sidewall height123.8 mm
Circumference2,373.5 mm
Revolutions / mile678.1
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 +0.99%. 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 | 40.4 km/h | 25 mph | 25 mph |
| 60 km/h | 60.6 km/h | 37 mph | 38 mph |
| 80 km/h | 80.8 km/h | 50 mph | 50 mph |
| 100 km/h | 101 km/h | 62 mph | 63 mph |
| 120 km/h | 121.2 km/h | 75 mph | 75 mph |
See individual specs: 275/45R20 · 255/50R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.