345/45R20 vs 325/45R20
Going from one to the other changes the overall height by -2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/45R20 | 325/45R20 | Change |
|---|---|---|---|
| Overall height | 32.2" | 31.5" | -2.2 % |
| Sidewall | 6.11" | 5.76" | -5.8 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 101.2" | 99.0" | -2.2 % |
| Revs per mile | 626 | 640 | +2.2 % |
| Axle height | 16.11" | 15.76" | -9.0 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 345/45R20. Fit 325/45R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.5 mph | -1.5 |
| 90 mph | 88.0 mph | -2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,780 on the road, so it over-counts by 220. At a true 70 mph the first turns 730 times a minute, the second 747.