315/75R22.5 vs 305/80R22.5
Going from one to the other changes the overall height by +1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/75R22.5 | 305/80R22.5 | Change |
|---|---|---|---|
| Overall height | 41.1" | 41.7" | +1.5 % |
| Sidewall | 9.30" | 9.61" | +3.3 % |
| Section width | 315 mm | 305 mm | -3.2 % |
| Circumference | 129.1" | 131.0" | +1.5 % |
| Revs per mile | 491 | 484 | -1.5 % |
| Axle height | 20.55" | 20.86" | +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 315/75R22.5. Fit 305/80R22.5 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 10,148 on the road, so it under-counts by 148. At a true 70 mph the first turns 572 times a minute, the second 564.