305/50R21 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 | 305/50R21 | 325/50R21 | Change |
|---|---|---|---|
| Overall height | 33.0" | 33.8" | +2.4 % |
| Sidewall | 6.00" | 6.40" | +6.6 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 103.7" | 106.2" | +2.4 % |
| Revs per mile | 611 | 597 | -2.4 % |
| Axle height | 16.50" | 16.90" | +10.0 mm |
| Approved rim width | 8.5–11" | 9–12" | overlap 9–11" |
Both are approved on a rim 9" to 11" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 305/50R21. Fit 325/50R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.7 mph | +1.7 |
| 90 mph | 92.1 mph | +2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,239 on the road, so it under-counts by 239. At a true 70 mph the first turns 713 times a minute, the second 696.