235/80R18 vs 255/75R18
Going from one to the other changes the overall height by +0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/80R18 | 255/75R18 | Change |
|---|---|---|---|
| Overall height | 32.8" | 33.1" | +0.8 % |
| Sidewall | 7.40" | 7.53" | +1.7 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 103.1" | 103.9" | +0.8 % |
| Revs per mile | 615 | 610 | -0.8 % |
| Axle height | 16.40" | 16.53" | +3.3 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 235/80R18. Fit 255/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.4 mph | +0.4 |
| 70 mph | 70.5 mph | +0.5 |
| 90 mph | 90.7 mph | +0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,078 on the road, so it under-counts by 78. At a true 70 mph the first turns 717 times a minute, the second 712.