+1.77%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 295/35R21, fitting 245/50R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.7 km/h | 25 mph | 25 mph |
| 60 km/h | 61.1 km/h | 37 mph | 38 mph |
| 80 km/h | 81.4 km/h | 50 mph | 51 mph |
| 100 km/h | 101.8 km/h | 62 mph | 63 mph |
| 120 km/h | 122.1 km/h | 75 mph | 76 mph |
Full comparison
295/35R21
Overall diameter739.9 mm · 29.13 in
Section width295 mm · 11.61 in
Sidewall height103.3 mm · 4.06 in
Circumference2324.5 mm · 91.5 in
Revolutions per mile692
Revolutions per km430
245/50R20
Overall diameter753.0 mm · 29.65 in
Section width245 mm · 9.65 in
Sidewall height122.5 mm · 4.82 in
Circumference2365.6 mm · 93.1 in
Revolutions per mile680
Revolutions per km423
+1.77%Safe swap — speedometer error is minor
Original
Overall diameter739.9 mm · 29.13 in
Sidewall height103.3 mm
Circumference2,324.5 mm
Revolutions / mile692.4
New
Overall diameter753 mm · 29.65 in
Sidewall height122.5 mm
Circumference2,365.6 mm
Revolutions / mile680.3
Speedometer & odometer error
The new tire changes overall diameter by +1.77%. 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 | 40.7 km/h | 25 mph | 25 mph |
| 60 km/h | 61.1 km/h | 37 mph | 38 mph |
| 80 km/h | 81.4 km/h | 50 mph | 51 mph |
| 100 km/h | 101.8 km/h | 62 mph | 63 mph |
| 120 km/h | 122.1 km/h | 75 mph | 76 mph |
See individual specs: 295/35R21 · 245/50R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.