335/60R21 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 | 335/60R21 | 325/60R21 | Change |
|---|---|---|---|
| Overall height | 36.8" | 36.4" | -1.3 % |
| Sidewall | 7.91" | 7.68" | -3.0 % |
| Section width | 335 mm | 325 mm | -3.0 % |
| Circumference | 115.7" | 114.2" | -1.3 % |
| Revs per mile | 548 | 555 | +1.3 % |
| Axle height | 18.41" | 18.18" | -6.0 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 335/60R21. Fit 325/60R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.1 mph | -0.9 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,872 on the road, so it over-counts by 128. At a true 70 mph the first turns 639 times a minute, the second 647.