+2.53%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 255/40R19, fitting 245/40R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 41.0 km/h | 25 mph | 25 mph |
| 60 km/h | 61.5 km/h | 37 mph | 38 mph |
| 80 km/h | 82.0 km/h | 50 mph | 51 mph |
| 100 km/h | 102.5 km/h | 62 mph | 64 mph |
| 120 km/h | 123.0 km/h | 75 mph | 76 mph |
Full comparison
255/40R19
Overall diameter686.6 mm · 27.03 in
Section width255 mm · 10.04 in
Sidewall height102.0 mm · 4.02 in
Circumference2157.0 mm · 84.9 in
Revolutions per mile746
Revolutions per km464
245/40R20
Overall diameter704.0 mm · 27.72 in
Section width245 mm · 9.65 in
Sidewall height98.0 mm · 3.86 in
Circumference2211.7 mm · 87.1 in
Revolutions per mile728
Revolutions per km452
+2.53%Safe swap — speedometer error is minor
Original
Overall diameter686.6 mm · 27.03 in
Sidewall height102 mm
Circumference2,157 mm
Revolutions / mile746.1
New
Overall diameter704 mm · 27.72 in
Sidewall height98 mm
Circumference2,211.7 mm
Revolutions / mile727.7
Speedometer & odometer error
The new tire changes overall diameter by +2.53%. 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 | 25 mph |
| 60 km/h | 61.5 km/h | 37 mph | 38 mph |
| 80 km/h | 82 km/h | 50 mph | 51 mph |
| 100 km/h | 102.5 km/h | 62 mph | 64 mph |
| 120 km/h | 123 km/h | 75 mph | 76 mph |
See individual specs: 255/40R19 · 245/40R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.