325/40R22 vs 315/45R22
Going from one to the other changes the overall height by +2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/40R22 | 315/45R22 | Change |
|---|---|---|---|
| Overall height | 32.2" | 33.2" | +2.9 % |
| Sidewall | 5.12" | 5.58" | +9.0 % |
| Section width | 325 mm | 315 mm | -3.1 % |
| Circumference | 101.3" | 104.2" | +2.9 % |
| Revs per mile | 626 | 608 | -2.9 % |
| Axle height | 16.12" | 16.58" | +11.8 mm |
| Approved rim width | 9–12" | 8.5–11.5" | overlap 9–11.5" |
Both are approved on a rim 9" to 11.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 325/40R22. Fit 315/45R22 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 72.0 mph | +2.0 |
| 90 mph | 92.6 mph | +2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,287 on the road, so it under-counts by 287. At a true 70 mph the first turns 730 times a minute, the second 710.