325/40R21 vs 315/45R21
Going from one to the other changes the overall height by +3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/40R21 | 315/45R21 | Change |
|---|---|---|---|
| Overall height | 31.2" | 32.2" | +3.0 % |
| Sidewall | 5.12" | 5.58" | +9.0 % |
| Section width | 325 mm | 315 mm | -3.1 % |
| Circumference | 98.1" | 101.0" | +3.0 % |
| Revs per mile | 646 | 627 | -3.0 % |
| Axle height | 15.62" | 16.08" | +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 325/40R21. Fit 315/45R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.5 mph | +1.5 |
| 70 mph | 72.1 mph | +2.1 |
| 90 mph | 92.7 mph | +2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,296 on the road, so it under-counts by 296. At a true 70 mph the first turns 753 times a minute, the second 732.