255/70R24 vs 245/75R24
Going from one to the other changes the overall height by +1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/70R24 | 245/75R24 | Change |
|---|---|---|---|
| Overall height | 38.1" | 38.5" | +1.1 % |
| Sidewall | 7.03" | 7.23" | +2.9 % |
| Section width | 255 mm | 245 mm | -3.9 % |
| Circumference | 119.6" | 120.9" | +1.1 % |
| Revs per mile | 530 | 524 | -1.1 % |
| Axle height | 19.03" | 19.23" | +5.3 mm |
| Approved rim width | 7–9.5" | 7–9" | overlap 7–9" |
Both are approved on a rim 7" to 9" 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 245/75R24 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.0 mph | +1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,109 on the road, so it under-counts by 109. At a true 70 mph the first turns 618 times a minute, the second 612.