325/55R20 vs 345/50R20
Going from one to the other changes the overall height by -1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/55R20 | 345/50R20 | Change |
|---|---|---|---|
| Overall height | 34.1" | 33.6" | -1.4 % |
| Sidewall | 7.04" | 6.79" | -3.5 % |
| Section width | 325 mm | 345 mm | +6.2 % |
| Circumference | 107.0" | 105.5" | -1.4 % |
| Revs per mile | 592 | 601 | +1.4 % |
| Axle height | 17.04" | 16.79" | -6.3 mm |
| Approved rim width | 9–12" | 9.5–12.5" | 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 325/55R20. Fit 345/50R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.7 mph | -1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,856 on the road, so it over-counts by 144. At a true 70 mph the first turns 691 times a minute, the second 701.