Unfortunately my expertise does not lend itself to this conversation, however, I have $0.02 to add...
Top Gear did a couple of great fuel segments (that I know of thanks to YouTube).
1st: They took an Audi A8 TDI and attempted to get 800 miles out of 1 tank of gas. He claims that coasting in-gear opposed to in neutral uses no gas, whereas in neutral the engine has to keep itself idling.
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2nd: