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Speed is a problem. If you already have to push about half etc much gas in normal driving you engine already is at good fuel efficiency level in a BSFC map. So there is no room for improvement in fuel consumtion average if glides become shorter than pushes or about the same. Usually if you have to press the gas already hard the engine dont waste the fuel so much and you can predict that car will not coast well in those speeds.
Engine off will help but if the glides are still very short effect can be negative, every time you make a bumb start you brake little. Also its not fun to start a push every five seconds etc.
My advice is that you can test how high speeds you will see better results if you accelerate at level road from 30 to 80 MPH and start coasting in neutral. If lose that each MPH faster than you have gained in acceleration it will not work well. As soon as times start to be so that you can coast so that it take about 1.5 times longer to slow down than it takes to accelerate you start to see some real gains. Best to take it to video and watch it in slow motion etc to be able to tell for sure.
In a downhills it can be done at little higher speed and uphills much slower, if at all.
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