345/55R20 vs 325/55R20
Going from one to the other changes the overall height by -2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/55R20 | 325/55R20 | Change |
|---|---|---|---|
| Overall height | 34.9" | 34.1" | -2.5 % |
| Sidewall | 7.47" | 7.04" | -5.8 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 109.8" | 107.0" | -2.5 % |
| Revs per mile | 577 | 592 | +2.5 % |
| Axle height | 17.47" | 17.04" | -11.0 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 345/55R20. Fit 325/55R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 70 mph | 68.3 mph | -1.7 |
| 90 mph | 87.8 mph | -2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,752 on the road, so it over-counts by 248. At a true 70 mph the first turns 673 times a minute, the second 691.