345/50R20 vs 355/45R20
Going from one to the other changes the overall height by -3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/50R20 | 355/45R20 | Change |
|---|---|---|---|
| Overall height | 33.6" | 32.6" | -3.0 % |
| Sidewall | 6.79" | 6.29" | -7.4 % |
| Section width | 345 mm | 355 mm | +2.9 % |
| Circumference | 105.5" | 102.3" | -3.0 % |
| Revs per mile | 601 | 619 | +3.0 % |
| Axle height | 16.79" | 16.29" | -12.8 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 345/50R20. Fit 355/45R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.3 mph | -2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,701 on the road, so it over-counts by 299. At a true 70 mph the first turns 701 times a minute, the second 722.