315/60R21 vs 325/60R21
Going from one to the other changes the overall height by +1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/60R21 | 325/60R21 | Change |
|---|---|---|---|
| Overall height | 35.9" | 36.4" | +1.3 % |
| Sidewall | 7.44" | 7.68" | +3.2 % |
| Section width | 315 mm | 325 mm | +3.2 % |
| Circumference | 112.7" | 114.2" | +1.3 % |
| Revs per mile | 562 | 555 | -1.3 % |
| Axle height | 17.94" | 18.18" | +6.0 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 315/60R21. Fit 325/60R21 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 | 70.9 mph | +0.9 |
| 90 mph | 91.2 mph | +1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,132 on the road, so it under-counts by 132. At a true 70 mph the first turns 656 times a minute, the second 647.