305/70R22.5 vs 315/65R22.5
Going from one to the other changes the overall height by -1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/70R22.5 | 315/65R22.5 | Change |
|---|---|---|---|
| Overall height | 39.3" | 38.6" | -1.8 % |
| Sidewall | 8.41" | 8.06" | -4.1 % |
| Section width | 305 mm | 315 mm | +3.3 % |
| Circumference | 123.5" | 121.3" | -1.8 % |
| Revs per mile | 513 | 522 | +1.8 % |
| Axle height | 19.66" | 19.31" | -8.8 mm |
| Approved rim width | 8.5–11" | 8.5–11.5" | 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 305/70R22.5. Fit 315/65R22.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.8 mph | -1.2 |
| 90 mph | 88.4 mph | -1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,825 on the road, so it over-counts by 175. At a true 70 mph the first turns 599 times a minute, the second 609.