5 Speed Gear ratio advice?

Probably the novice question here... how do you determine what the best torque is for cruising

You would have to look at the whole build but everything mechanical has a sweet spot where you get the most for the least.Like Rumble explained your camshafts' operating range plays a big part.If it operates from 2500 - 6000 rpm it would struggle cruising at 2200 rpm.