I don't think mpg is as related to rpm on a diesel as it is on a car. It takes a set amount of POWER to move the car x distance at y speed based on the rolling resistance of the tires and aeordynamic drag. That power requirement when factored with the efficiency of the engine gives fuel economy.
Increased resistance in any way (dragging brakes, added roof racks, who knows what else) will decrease economy, so will decreasing the efficiency of the engine with things like bad thermostats.

__________________
Sadly Benz-less