345/55R20 vs 355/50R20
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/55R20 | 355/50R20 | Change |
|---|---|---|---|
| Overall height | 34.9" | 34.0" | -2.8 % |
| Sidewall | 7.47" | 6.99" | -6.5 % |
| Section width | 345 mm | 355 mm | +2.9 % |
| Circumference | 109.8" | 106.7" | -2.8 % |
| Revs per mile | 577 | 594 | +2.8 % |
| Axle height | 17.47" | 16.99" | -12.3 mm |
| Approved rim width | 9.5–12.5" | 10–13" | overlap 10–12.5" |
Both are approved on a rim 10" to 12.5" 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 355/50R20 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 | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,724 on the road, so it over-counts by 276. At a true 70 mph the first turns 673 times a minute, the second 693.