315/80R24.5 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 | 315/80R24.5 | 305/80R24.5 | Change |
|---|---|---|---|
| Overall height | 44.3" | 43.7" | -1.4 % |
| Sidewall | 9.92" | 9.61" | -3.2 % |
| Section width | 315 mm | 305 mm | -3.2 % |
| Circumference | 139.3" | 137.3" | -1.4 % |
| Revs per mile | 455 | 461 | +1.4 % |
| Axle height | 22.17" | 21.86" | -8.0 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 315/80R24.5. Fit 305/80R24.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.7 mph | -1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,858 on the road, so it over-counts by 142. At a true 70 mph the first turns 531 times a minute, the second 538.