235/40R18 vs 255/40R18
Going from one to the other changes the overall height by +2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/40R18 | 255/40R18 | Change |
|---|---|---|---|
| Overall height | 25.4" | 26.0" | +2.5 % |
| Sidewall | 3.70" | 4.02" | +8.5 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 79.8" | 81.8" | +2.5 % |
| Revs per mile | 794 | 775 | -2.5 % |
| Axle height | 12.70" | 13.02" | +8.0 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 235/40R18. Fit 255/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.7 mph | +1.7 |
| 90 mph | 92.2 mph | +2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,248 on the road, so it under-counts by 248. At a true 70 mph the first turns 926 times a minute, the second 904.