235/75R18 vs 255/65R18
Going from one to the other changes the overall height by -2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/75R18 | 255/65R18 | Change |
|---|---|---|---|
| Overall height | 31.9" | 31.1" | -2.6 % |
| Sidewall | 6.94" | 6.53" | -6.0 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 100.1" | 97.6" | -2.6 % |
| Revs per mile | 633 | 650 | +2.6 % |
| Axle height | 15.94" | 15.53" | -10.5 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 235/75R18. Fit 255/65R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,741 on the road, so it over-counts by 259. At a true 70 mph the first turns 738 times a minute, the second 758.
What drivers reported
Nobody has filed a complaint naming 235/75R18. 255/65R18 carries 24.