255/70R24 vs 255/75R24
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 | 255/70R24 | 255/75R24 | Change |
|---|---|---|---|
| Overall height | 38.1" | 39.1" | +2.6 % |
| Sidewall | 7.03" | 7.53" | +7.1 % |
| Section width | 255 mm | 255 mm | 0.0 % |
| Circumference | 119.6" | 122.7" | +2.6 % |
| Revs per mile | 530 | 516 | -2.6 % |
| Axle height | 19.03" | 19.53" | +12.8 mm |
| Approved rim width | 7–9.5" | 7–9.5" | overlap 7–9.5" |
Both are approved on a rim 7" to 9.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 255/70R24. Fit 255/75R24 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,264 on the road, so it under-counts by 264. At a true 70 mph the first turns 618 times a minute, the second 602.