325/35R21 vs 345/30R21
Going from one to the other changes the overall height by -2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/35R21 | 345/30R21 | Change |
|---|---|---|---|
| Overall height | 30.0" | 29.1" | -2.7 % |
| Sidewall | 4.48" | 4.07" | -9.0 % |
| Section width | 325 mm | 345 mm | +6.2 % |
| Circumference | 94.1" | 91.6" | -2.7 % |
| Revs per mile | 673 | 692 | +2.7 % |
| Axle height | 14.98" | 14.57" | -10.3 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 325/35R21. Fit 345/30R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.1 mph | -1.9 |
| 90 mph | 87.6 mph | -2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,731 on the road, so it over-counts by 269. At a true 70 mph the first turns 785 times a minute, the second 807.