In the old days I would explain the math, but why bother these days? There are any number of gear ratio calculators online if you google it. here is one:
gear ratio calculator
You need your tire size, overdrive ratio for the gear you will be running in, your target rpm, and the speed you want to run at, and the tool will calculate the gear ratio. Maybe some of the diesel guys out there can help with the most efficient cruising rpm for your engine?