315/55R21 vs 325/50R21
Going from one to the other changes the overall height by -2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/55R21 | 325/50R21 | Change |
|---|---|---|---|
| Overall height | 34.6" | 33.8" | -2.4 % |
| Sidewall | 6.82" | 6.40" | -6.2 % |
| Section width | 315 mm | 325 mm | +3.2 % |
| Circumference | 108.8" | 106.2" | -2.4 % |
| Revs per mile | 582 | 597 | +2.4 % |
| Axle height | 17.32" | 16.90" | -10.8 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 315/55R21. Fit 325/50R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 70 mph | 68.3 mph | -1.7 |
| 90 mph | 87.8 mph | -2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,756 on the road, so it over-counts by 244. At a true 70 mph the first turns 679 times a minute, the second 696.