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The answer to your question is below.
I just finished some research on this exact topic. My findings are published here:
kc7ekk-solar.blogspot.com/2011/04/coast-in-neutral-or-in-gear.html
The answer is both.
With a cold engine, coasting in neutral uses 12% less fuel than coasting in gear. (at least for the default values in the computer of my wife's 2002 Chevy Venture). When the engine is warmed up, it flips the other way and coasting in gear saves on average 2.7% more fuel than coasting in neutral.
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