255/65R15 vs 235/75R15
Going from one to the other changes the overall height by +2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/65R15 | 235/75R15 | Change |
|---|---|---|---|
| Overall height | 28.1" | 28.9" | +2.9 % |
| Sidewall | 6.53" | 6.94" | +6.3 % |
| Section width | 255 mm | 235 mm | -7.8 % |
| Circumference | 88.1" | 90.7" | +2.9 % |
| Revs per mile | 719 | 698 | -2.9 % |
| Axle height | 14.03" | 14.44" | +10.5 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 255/65R15. Fit 235/75R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.5 mph | +1.5 |
| 70 mph | 72.1 mph | +2.1 |
| 90 mph | 92.7 mph | +2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,295 on the road, so it under-counts by 295. At a true 70 mph the first turns 839 times a minute, the second 815.
What drivers reported
255/65R15 carries 1 complaints. 235/75R15 carries 219 and 3 deaths.