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I have no means to test the two scenarios in your original post, so hard data is not available to me. What I do have is a 45+ MPG average for close to 6k miles that shows I am fairly economical with my Fiesta.
I don't worry much about "challenges" from anyone.
What I do know is that the basic principle is the conservation of momentum. If I coast in neutral at an average speed of 50 MPH then I am using the fuel consumption at idle to keep my engine running which would be more then if I coasted with the engine off.
With the engine running I have no fear of a catastrophic transmission failure due to lack of lubrication.
My strategy is to coast in neutral (engine running) while getting well over a hundred MPG to the greatest extent possible. When outside influences force me to decelerate at a greater rate, something I try to avoid like the plague, then I shift back into drive and let engine braking slow me down at a faster rate. When that fails to provide the necessary deceleration, I am forced to apply the brakes which turns my precious inertia into heat and brake dust.
Whether my technique is more or less efficient is based on my understanding of the fact that having the engine bleed off vehicle speed or idling at a consumption rate of about .15 GPH depends on my failure to prefectly anticipate every deceleration event I may encounter in a round trip of 40 miles with 46 traffic lights that could force me to stop, instead of 40 miles of 45 MPH highway where I would never have to stop.
I guess in the final analysis it really depends on your situation. If stopping is unavoidable the use engine braking, but if you have the means to anticipate and prevent the deceleration events in the first place then engine drag deceleration would be avoidable as well as friction braking. If I had the ability to wipe the 46 traffic lights off of my drive, and eliminate all the traffic that can force me to stop, I probably could get 60 MPG instead of 45.
While the argument that using no fuel versus having the engine idle while coasting in neutral has some validity, the most relevant fact is your driving environment, which can force you to take actions that are unavoidable. Without actually driving your normal route, it is impossible for me to develop a strategy to maximise fuel consumption in your particular scenario.
Give me a country road with shallow hills and no traffic to disturb and 70+ MPG would be a potential result. Sadly my wife needs to be close to a Wal-Mart to be truly content.
regards
Mech
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