335/50R21 vs 325/50R21
Going from one to the other changes the overall height by -1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 335/50R21 | 325/50R21 | Change |
|---|---|---|---|
| Overall height | 34.2" | 33.8" | -1.2 % |
| Sidewall | 6.59" | 6.40" | -3.0 % |
| Section width | 335 mm | 325 mm | -3.0 % |
| Circumference | 107.4" | 106.2" | -1.2 % |
| Revs per mile | 590 | 597 | +1.2 % |
| Axle height | 17.09" | 16.90" | -5.0 mm |
| Approved rim width | 9.5–12.5" | 9–12" | overlap 9.5–12" |
Both are approved on a rim 9.5" to 12" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 335/50R21. Fit 325/50R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.2 mph | -0.8 |
| 90 mph | 89.0 mph | -1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,885 on the road, so it over-counts by 115. At a true 70 mph the first turns 688 times a minute, the second 696.