-0.49%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 235/55R19, fitting 255/45R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.8 km/h | 25 mph | 25 mph |
| 60 km/h | 59.7 km/h | 37 mph | 37 mph |
| 80 km/h | 79.6 km/h | 50 mph | 49 mph |
| 100 km/h | 99.5 km/h | 62 mph | 62 mph |
| 120 km/h | 119.4 km/h | 75 mph | 74 mph |
Full comparison
235/55R19
Overall diameter741.1 mm · 29.18 in
Section width235 mm · 9.25 in
Sidewall height129.3 mm · 5.09 in
Circumference2328.2 mm · 91.7 in
Revolutions per mile691
Revolutions per km430
255/45R20
Overall diameter737.5 mm · 29.04 in
Section width255 mm · 10.04 in
Sidewall height114.8 mm · 4.52 in
Circumference2316.9 mm · 91.2 in
Revolutions per mile695
Revolutions per km432
-0.49%Safe swap — speedometer error is minor
Original
Overall diameter741.1 mm · 29.18 in
Sidewall height129.3 mm
Circumference2,328.2 mm
Revolutions / mile691.2
New
Overall diameter737.5 mm · 29.04 in
Sidewall height114.8 mm
Circumference2,316.9 mm
Revolutions / mile694.6
Speedometer & odometer error
The new tire changes overall diameter by -0.49%. 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 | 39.8 km/h | 25 mph | 25 mph |
| 60 km/h | 59.7 km/h | 37 mph | 37 mph |
| 80 km/h | 79.6 km/h | 50 mph | 49 mph |
| 100 km/h | 99.5 km/h | 62 mph | 62 mph |
| 120 km/h | 119.4 km/h | 75 mph | 74 mph |
See individual specs: 235/55R19 · 255/45R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.