-6.71%Not recommended — recalibrate the speedometer
Speedometer error
With a speedometer calibrated to 315/70R17, fitting 255/70R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 37.3 km/h | 25 mph | 23 mph |
| 60 km/h | 56.0 km/h | 37 mph | 35 mph |
| 80 km/h | 74.6 km/h | 50 mph | 46 mph |
| 100 km/h | 93.3 km/h | 62 mph | 58 mph |
| 120 km/h | 111.9 km/h | 75 mph | 70 mph |
Full comparison
315/70R17
Overall diameter872.8 mm · 34.36 in
Section width315 mm · 12.40 in
Sidewall height220.5 mm · 8.68 in
Circumference2742.0 mm · 108.0 in
Revolutions per mile587
Revolutions per km365
255/70R18
Overall diameter814.2 mm · 32.06 in
Section width255 mm · 10.04 in
Sidewall height178.5 mm · 7.03 in
Circumference2557.9 mm · 100.7 in
Revolutions per mile629
Revolutions per km391
-6.71%Not recommended — recalibrate the speedometer
Original
Overall diameter872.8 mm · 34.36 in
Sidewall height220.5 mm
Circumference2,742 mm
Revolutions / mile586.9
New
Overall diameter814.2 mm · 32.06 in
Sidewall height178.5 mm
Circumference2,557.9 mm
Revolutions / mile629.2
Speedometer & odometer error
The new tire changes overall diameter by -6.71%. 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 | 37.3 km/h | 25 mph | 23 mph |
| 60 km/h | 56 km/h | 37 mph | 35 mph |
| 80 km/h | 74.6 km/h | 50 mph | 46 mph |
| 100 km/h | 93.3 km/h | 62 mph | 58 mph |
| 120 km/h | 111.9 km/h | 75 mph | 70 mph |
See individual specs: 315/70R17 · 255/70R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.