345/40R20 vs 325/45R20
Going from one to the other changes the overall height by +2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/40R20 | 325/45R20 | Change |
|---|---|---|---|
| Overall height | 30.9" | 31.5" | +2.1 % |
| Sidewall | 5.43" | 5.76" | +6.0 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 97.0" | 99.0" | +2.1 % |
| Revs per mile | 653 | 640 | -2.1 % |
| Axle height | 15.43" | 15.76" | +8.3 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 345/40R20. Fit 325/45R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.1 mph | +1.1 |
| 70 mph | 71.5 mph | +1.5 |
| 90 mph | 91.9 mph | +1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,210 on the road, so it under-counts by 210. At a true 70 mph the first turns 762 times a minute, the second 747.