According to my Scanguage II, my 3.0l Mitsubishi uses about .45 gph idling in drive once warm. In Neutral it uses 0.35 gph.
My initial thought on the subject is that by slowing down the engine it would use less fuel. Which is probably applicable to carburators, not sure.
New cars with Idle Air Control valves try to keep a constant idle rpm. So when you load the engine down in gear, it opens a small secondary throttle & gives it more go juice.
This is only my thoughts, if they are wrong, someone please correct me.
Don
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