I've been trying a bit of P&G with my diesel Golf for a while. I find that on my 7 mile run (partly downhill) into Cambridge I could, if careful, get as much as 93 -100 mpg.
But I've just tried an experiment with a fuel cut-off so that I can stop the engine while leaving the ignition circuit on and am very happy with the results!
I got an indicated 145mpg going into town - and 119mpg coming back ('up hill')!!
I really didn't expect it to make that big a difference over 'engine on coasting'....
I just have a couple of wires and a toggle switch in series with the fuel solenoid on the injector pump for the moment, and it seems to work very well. The engine dies almost immediately - maybe takes 2 seconds to drop to zero revs.
Bump starting in 5th seems to be the smoothest way to restart.
Now I will have to make something more permanent, so am planning on a normally-closed relay in the engine compartment, and a simple push-switch (normally open -push to close) on the dashboard to earth one end of the solenoid coil in order to open the circuit. Hopefully the result will be reasonably fail-safe so long as all connections are made securely (soldered) and the wiring is routed carefully.
BTW This is a NON-turbo car I'm talking about