325/45R21 vs 335/40R21
Going from one to the other changes the overall height by -3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/45R21 | 335/40R21 | Change |
|---|---|---|---|
| Overall height | 32.5" | 31.6" | -3.0 % |
| Sidewall | 5.76" | 5.28" | -8.4 % |
| Section width | 325 mm | 335 mm | +3.1 % |
| Circumference | 102.2" | 99.1" | -3.0 % |
| Revs per mile | 620 | 639 | +3.0 % |
| Axle height | 16.26" | 15.78" | -12.3 mm |
| Approved rim width | 9–12" | 9.5–12.5" | 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 325/45R21. Fit 335/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.3 mph | -2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,703 on the road, so it over-counts by 297. At a true 70 mph the first turns 724 times a minute, the second 746.