345/30R21 vs 325/35R21
Going from one to the other changes the overall height by +2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/30R21 | 325/35R21 | Change |
|---|---|---|---|
| Overall height | 29.1" | 30.0" | +2.8 % |
| Sidewall | 4.07" | 4.48" | +9.9 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 91.6" | 94.1" | +2.8 % |
| Revs per mile | 692 | 673 | -2.8 % |
| Axle height | 14.57" | 14.98" | +10.3 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 345/30R21. Fit 325/35R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 71.9 mph | +1.9 |
| 90 mph | 92.5 mph | +2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,277 on the road, so it under-counts by 277. At a true 70 mph the first turns 807 times a minute, the second 785.