+1.33%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 245/45R18, fitting 255/45R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.5 km/h | 25 mph | 25 mph |
| 60 km/h | 60.8 km/h | 37 mph | 38 mph |
| 80 km/h | 81.1 km/h | 50 mph | 50 mph |
| 100 km/h | 101.3 km/h | 62 mph | 63 mph |
| 120 km/h | 121.6 km/h | 75 mph | 76 mph |
Full comparison
245/45R18
Overall diameter677.7 mm · 26.68 in
Section width245 mm · 9.65 in
Sidewall height110.3 mm · 4.34 in
Circumference2129.1 mm · 83.8 in
Revolutions per mile756
Revolutions per km470
255/45R18
Overall diameter686.7 mm · 27.04 in
Section width255 mm · 10.04 in
Sidewall height114.8 mm · 4.52 in
Circumference2157.3 mm · 84.9 in
Revolutions per mile746
Revolutions per km464
+1.33%Safe swap — speedometer error is minor
Original
Overall diameter677.7 mm · 26.68 in
Sidewall height110.3 mm
Circumference2,129.1 mm
Revolutions / mile755.9
New
Overall diameter686.7 mm · 27.04 in
Sidewall height114.8 mm
Circumference2,157.3 mm
Revolutions / mile746
Speedometer & odometer error
The new tire changes overall diameter by +1.33%. 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 | 40.5 km/h | 25 mph | 25 mph |
| 60 km/h | 60.8 km/h | 37 mph | 38 mph |
| 80 km/h | 81.1 km/h | 50 mph | 50 mph |
| 100 km/h | 101.3 km/h | 62 mph | 63 mph |
| 120 km/h | 121.6 km/h | 75 mph | 76 mph |
See individual specs: 245/45R18 · 255/45R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.