315/70R22.5 vs 305/75R22.5
Going from one to the other changes the overall height by +1.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/70R22.5 | 305/75R22.5 | Change |
|---|---|---|---|
| Overall height | 39.9" | 40.5" | +1.6 % |
| Sidewall | 8.68" | 9.01" | +3.7 % |
| Section width | 315 mm | 305 mm | -3.2 % |
| Circumference | 125.2" | 127.3" | +1.6 % |
| Revs per mile | 506 | 498 | -1.6 % |
| Axle height | 19.93" | 20.26" | +8.3 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 315/70R22.5. Fit 305/75R22.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.1 mph | +1.1 |
| 90 mph | 91.5 mph | +1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,163 on the road, so it under-counts by 163. At a true 70 mph the first turns 590 times a minute, the second 581.