315/70R24.5 vs 315/75R24.5
Going from one to the other changes the overall height by +3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/70R24.5 | 315/75R24.5 | Change |
|---|---|---|---|
| Overall height | 41.9" | 43.1" | +3.0 % |
| Sidewall | 8.68" | 9.30" | +7.1 % |
| Section width | 315 mm | 315 mm | 0.0 % |
| Circumference | 131.5" | 135.4" | +3.0 % |
| Revs per mile | 482 | 468 | -3.0 % |
| Axle height | 20.93" | 21.55" | +15.8 mm |
| Approved rim width | 8.5–11.5" | 8.5–11.5" | overlap 8.5–11.5" |
Both are approved on a rim 8.5" to 11.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 315/70R24.5. Fit 315/75R24.5 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 10,296 on the road, so it under-counts by 296. At a true 70 mph the first turns 562 times a minute, the second 546.