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You spec a different final drive ratio to end up at the same rpm. 10 years ago our semi trucks that ran direct drive had 2.47 rear ends. I think some of the new Freightliners run 2.16 final drives!
From a fleet management perspective, I found that you wanted your transmission to be in direct drive at whatever speed you consumed most of your fuel. For flat land and cross country highway trucks, that was top gear. But for some local applications, especially those climbing grades, you would want them to cruise the highway in overdrive and be in direct when they were chugging up Donner summit.
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