255/70R15 vs 235/75R15
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 | 255/70R15 | 235/75R15 | Change |
|---|---|---|---|
| Overall height | 29.1" | 28.9" | -0.6 % |
| Sidewall | 7.03" | 6.94" | -1.3 % |
| Section width | 255 mm | 235 mm | -7.8 % |
| Circumference | 91.3" | 90.7" | -0.6 % |
| Revs per mile | 694 | 698 | +0.6 % |
| Axle height | 14.53" | 14.44" | -2.3 mm |
| Approved rim width | 7–9.5" | 6.5–8.5" | 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 255/70R15. Fit 235/75R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.7 mph | -0.3 |
| 70 mph | 69.6 mph | -0.4 |
| 90 mph | 89.5 mph | -0.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,939 on the road, so it over-counts by 61. At a true 70 mph the first turns 810 times a minute, the second 815.
What drivers reported
255/70R15 carries 13 complaints. 235/75R15 carries 219 and 3 deaths.