345/55R20 vs 325/60R20
Going from one to the other changes the overall height by +1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/55R20 | 325/60R20 | Change |
|---|---|---|---|
| Overall height | 34.9" | 35.4" | +1.2 % |
| Sidewall | 7.47" | 7.68" | +2.8 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 109.8" | 111.1" | +1.2 % |
| Revs per mile | 577 | 570 | -1.2 % |
| Axle height | 17.47" | 17.68" | +5.3 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 345/55R20. Fit 325/60R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.1 mph | +1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,118 on the road, so it under-counts by 118. At a true 70 mph the first turns 673 times a minute, the second 666.
What drivers reported
Nobody has filed a complaint naming 345/55R20. 325/60R20 carries 2.