235/85R18 vs 255/75R18
Going from one to the other changes the overall height by -2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/85R18 | 255/75R18 | Change |
|---|---|---|---|
| Overall height | 33.7" | 33.1" | -2.0 % |
| Sidewall | 7.86" | 7.53" | -4.3 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 106.0" | 103.9" | -2.0 % |
| Revs per mile | 598 | 610 | +2.0 % |
| Axle height | 16.86" | 16.53" | -8.5 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 235/85R18. Fit 255/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,802 on the road, so it over-counts by 198. At a true 70 mph the first turns 698 times a minute, the second 712.