The biggest problem here is that when you say “coasting in neutral” nearly everybody on this board assumes you are talking about freewheeling down some really long and steep mountain road and gives an answer based on that assumption. If I find myself in that situation, I keep my car in gear and let the DFCO feature cut off all fuel use. Ditto for approaching a red light when I have so much speed that I have to brake anyway, sometimes I even downshift to keep the car in DFCO longer.
My commute to work is filled with dozens of, oh, let’s not call them hills, let’s call them coasting opportunities. These are grades so mild that you don’t need to use your brakes when you freewheel down them. What happens when you keep your car in gear? DFCO does not come into the picture because if you take your foot off the gas, the engine braking slows you down and you need to re-accelerate. What happens is that you have to hold a small amount of throttle just so the engine will let the car coast. The fuel use difference is the difference between idle and a 3000 rpm fast idle. A Honda Fit uses about .3 gallons per hour idling, triple that idle speed and the fuel burn will be about triple the idle fuel burn.
Is it illegal? Cool! Another good reason to do it! Stick it to the man!
Does it save only a “teaspoon” of gas? My last tank yielded 46 mpg. How’s your gas mileage?