355/50R20 vs 345/55R20
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 | 355/50R20 | 345/55R20 | Change |
|---|---|---|---|
| Overall height | 34.0" | 34.9" | +2.8 % |
| Sidewall | 6.99" | 7.47" | +6.9 % |
| Section width | 355 mm | 345 mm | -2.8 % |
| Circumference | 106.7" | 109.8" | +2.8 % |
| Revs per mile | 594 | 577 | -2.8 % |
| Axle height | 16.99" | 17.47" | +12.3 mm |
| Approved rim width | 10–13" | 9.5–12.5" | 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 355/50R20. Fit 345/55R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 72.0 mph | +2.0 |
| 90 mph | 92.6 mph | +2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,284 on the road, so it under-counts by 284. At a true 70 mph the first turns 693 times a minute, the second 673.