325/30R20 vs 345/30R20
Going from one to the other changes the overall height by +1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/30R20 | 345/30R20 | Change |
|---|---|---|---|
| Overall height | 27.7" | 28.1" | +1.7 % |
| Sidewall | 3.84" | 4.07" | +6.2 % |
| Section width | 325 mm | 345 mm | +6.2 % |
| Circumference | 87.0" | 88.4" | +1.7 % |
| Revs per mile | 729 | 716 | -1.7 % |
| Axle height | 13.84" | 14.07" | +6.0 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 325/30R20. Fit 345/30R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.2 mph | +1.2 |
| 90 mph | 91.5 mph | +1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,171 on the road, so it under-counts by 171. At a true 70 mph the first turns 850 times a minute, the second 836.