235/60R18 vs 255/55R18
Going from one to the other changes the overall height by -0.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/60R18 | 255/55R18 | Change |
|---|---|---|---|
| Overall height | 29.1" | 29.0" | -0.2 % |
| Sidewall | 5.55" | 5.52" | -0.5 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 91.4" | 91.2" | -0.2 % |
| Revs per mile | 693 | 694 | +0.2 % |
| Axle height | 14.55" | 14.52" | -0.8 mm |
| Approved rim width | 6.5–8.5" | 7–9.5" | overlap 7–8.5" |
Both are approved on a rim 7" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 235/60R18. Fit 255/55R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.9 mph | -0.1 |
| 50 mph | 49.9 mph | -0.1 |
| 70 mph | 69.9 mph | -0.1 |
| 90 mph | 89.8 mph | -0.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,980 on the road, so it over-counts by 20. At a true 70 mph the first turns 809 times a minute, the second 810.