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To expand on how P&G works to help a diesel:
Consider accessory and frictional losses. If your engine is turning at (for example) 2000rpm, it doesn't matter if it's 20% loaded or 90% loaded, turning the water pump, alternator fan, oil pump, etc. use just about the same amount of energy. Friction goes up a bit with things like piston rings, but it isn't linear.
The way it works out in practice is, that you end up with fewer losses by running the engine half the time at twice the load, and coasting at idle or with the engine off the other half of the time. You're literally cutting all accessory losses in half.
Some modern hybrids do some of this automatically - they have electric water pumps which are not tied to engine speed, and turn themselves on and off as needed, and even the engine itself doesn't necessarily run all the time, but rather, may cycle on and off as the battery is filled and depleted, with the battery carrying the vehicle while the engine isn't running.
Taller gearing always helps with economy because slowing the engine down cuts accessory and other parasitic losses too.
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