315/75R24.5 vs 315/80R24.5
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 | 315/75R24.5 | 315/80R24.5 | Change |
|---|---|---|---|
| Overall height | 43.1" | 44.3" | +2.9 % |
| Sidewall | 9.30" | 9.92" | +6.7 % |
| Section width | 315 mm | 315 mm | 0.0 % |
| Circumference | 135.4" | 139.3" | +2.9 % |
| Revs per mile | 468 | 455 | -2.9 % |
| Axle height | 21.55" | 22.17" | +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 315/75R24.5. Fit 315/80R24.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 72.0 mph | +2.0 |
| 90 mph | 92.6 mph | +2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,288 on the road, so it under-counts by 288. At a true 70 mph the first turns 546 times a minute, the second 531.