I have a Millermatic 172, running Argon/co2 mix (75/25 I believe). I will be welding all 3/16 material on my next project (utility trailer). I currently have (.30) wire in the welder. For this project would I be better served to use (.35) wire. I have seen some charts and they seem to recommend this size wire for this thickness steel. Looking at the chart on the inside of the welder, it indicates that I drop to a 3 for voltage and 55 for wire speed (4/70) if I were to use the (.30) wire). Why the significant drop in voltage and would I be assured of better penetration if I increased it to 4? As an amateur, I am nervous about the strength of the welds. The steel I will be using is new with mill scale on it, which I will grind off.

Any suggestions would be greatly appreciated.