To answer your question, no it wont cut the fuel completley by taking your foot off. Your engine would splutter and die if the fuel cut out when you took your foot off. Unless you drive a Prius or something.... Where as you take it out of gear and coast this is the same as being stationary in the vehicle with the engine idling, the only difference is you are rolling and thus recording a MPH. I think the confusion is being applied in calculation of the MPG as you are moving and the computer is factoring in the MPH.