315/75R24.5 vs 305/80R24.5
Going from one to the other changes the overall height by +1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/75R24.5 | 305/80R24.5 | Change |
|---|---|---|---|
| Overall height | 43.1" | 43.7" | +1.4 % |
| Sidewall | 9.30" | 9.61" | +3.3 % |
| Section width | 315 mm | 305 mm | -3.2 % |
| Circumference | 135.4" | 137.3" | +1.4 % |
| Revs per mile | 468 | 461 | -1.4 % |
| Axle height | 21.55" | 21.86" | +7.8 mm |
| Approved rim width | 8.5–11.5" | 8.5–11" | overlap 8.5–11" |
Both are approved on a rim 8.5" to 11" 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 305/80R24.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.3 mph | +1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,142 on the road, so it under-counts by 142. At a true 70 mph the first turns 546 times a minute, the second 538.