325/35R21 vs 315/35R21
Going from one to the other changes the overall height by -0.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/35R21 | 315/35R21 | Change |
|---|---|---|---|
| Overall height | 30.0" | 29.7" | -0.9 % |
| Sidewall | 4.48" | 4.34" | -3.1 % |
| Section width | 325 mm | 315 mm | -3.1 % |
| Circumference | 94.1" | 93.2" | -0.9 % |
| Revs per mile | 673 | 679 | +0.9 % |
| Axle height | 14.98" | 14.84" | -3.5 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 325/35R21. Fit 315/35R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.4 mph | -0.6 |
| 90 mph | 89.2 mph | -0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,908 on the road, so it over-counts by 92. At a true 70 mph the first turns 785 times a minute, the second 793.