[Diesel_Dave]

Getting back to the original question, the slowest rpm at which you can be in top gear is determined by the amount of low-end torque that your engine has and the amount of power required to push the vehicle down the road. For unloaded turbodiesel trucks like mine and Big Dave's it's nearly impossible to lug the engine. With 300+ lb-ft of clutch engagement torque I can run 6th gear at idle speed (800 rpm). That's close to 50 hp, so that's plenty for unloaded cruising on level ground.