235/75R15 vs 255/70R15
Going from one to the other changes the overall height by +0.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/75R15 | 255/70R15 | Change |
|---|---|---|---|
| Overall height | 28.9" | 29.1" | +0.6 % |
| Sidewall | 6.94" | 7.03" | +1.3 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 90.7" | 91.3" | +0.6 % |
| Revs per mile | 698 | 694 | -0.6 % |
| Axle height | 14.44" | 14.53" | +2.3 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 235/75R15. Fit 255/70R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.3 mph | +0.3 |
| 70 mph | 70.4 mph | +0.4 |
| 90 mph | 90.6 mph | +0.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,061 on the road, so it under-counts by 61. At a true 70 mph the first turns 815 times a minute, the second 810.
What drivers reported
235/75R15 carries 219 complaints and 3 deaths. 255/70R15 carries 13.