245/55R18 vs 255/55R18
Going from one to the other changes the overall height by +1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/55R18 | 255/55R18 | Change |
|---|---|---|---|
| Overall height | 28.6" | 29.0" | +1.5 % |
| Sidewall | 5.31" | 5.52" | +4.1 % |
| Section width | 245 mm | 255 mm | +4.1 % |
| Circumference | 89.9" | 91.2" | +1.5 % |
| Revs per mile | 705 | 694 | -1.5 % |
| Axle height | 14.31" | 14.52" | +5.5 mm |
| Approved rim width | 7–9" | 7–9.5" | overlap 7–9" |
Both are approved on a rim 7" to 9" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 245/55R18. Fit 255/55R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.1 mph | +1.1 |
| 90 mph | 91.4 mph | +1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,151 on the road, so it under-counts by 151. At a true 70 mph the first turns 822 times a minute, the second 810.