345/65R20 vs 325/65R20
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 | 345/65R20 | 325/65R20 | Change |
|---|---|---|---|
| Overall height | 37.7" | 36.6" | -2.7 % |
| Sidewall | 8.83" | 8.32" | -5.8 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 118.3" | 115.1" | -2.7 % |
| Revs per mile | 536 | 551 | +2.7 % |
| Axle height | 18.83" | 18.32" | -13.0 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 345/65R20. Fit 325/65R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 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 9,728 on the road, so it over-counts by 272. At a true 70 mph the first turns 625 times a minute, the second 642.