Interesting that the discussion regarding getting better mileage driving at 45mph with the diesels. I've always heard keeping rpms under 1800 was best. Also learned later on my TDI that even lower is not normally damaging and even better for mileage especially when your using your engine gently increase your coasting. I have to watch the rpms to ensure my overworked cam lobes are getting enough oil, so I feel more comfortable staying above 1100 rpm even though I still have enough torque to pull at idle. I've also noticed this also applies to gas engines as well. My D150 is going along nice and easy at 45mph at just under 1800 rpms. Once you get going above that the gearing and the poor aero really start to work against you.
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