315/65R21 vs 325/65R21
Going from one to the other changes the overall height by +1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/65R21 | 325/65R21 | Change |
|---|---|---|---|
| Overall height | 37.1" | 37.6" | +1.4 % |
| Sidewall | 8.06" | 8.32" | +3.2 % |
| Section width | 315 mm | 325 mm | +3.2 % |
| Circumference | 116.6" | 118.2" | +1.4 % |
| Revs per mile | 543 | 536 | -1.4 % |
| Axle height | 18.56" | 18.82" | +6.5 mm |
| Approved rim width | 8.5–11.5" | 9–12" | 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 315/65R21. Fit 325/65R21 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 | 91.2 mph | +1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,138 on the road, so it under-counts by 138. At a true 70 mph the first turns 634 times a minute, the second 625.