Thanks to all those with positive and informative replies. I'm just trying learn how engines work. Maybe I should have used the wording "less fuel injected" rather than "save fuel" to prevent people telling me to go catch a bus. Safety comes first so I'll continue to do what I've always done, which is coast in gear, regardless of how much fuel is actually used or not.

I asked the same question to a fellow engineer who knows more about cars than I do and he did tried to do some research for me. He reversed engineered some of the EFI system on a R31 Skyline RB30 engine.

He said that on this ECU when your foot is not on the accelerator the Throttle Position Sensor detects your foot is off and then uses Fuel Cut / Fuel Recover tables to determine what revs to stop injecting (to prevent exhaust backfire) and then when to continue injecting again.

For this ECU there are two sets of tables for neutral and non-neutral injection when TPS is closed (accelerator off). I think from memory the non-neutral may have slightly richer injector settings (especially for autos where load is put on the engine from the torque converter)

He sent me the graphed results for fuel cut and fuel recover. The results are rpm vs against temperature. They show non 0 values, but since he didn't pull the injection amount from the ECU to display it i can't be sure that is the case. From my workplace photobucket and webstorage is blocked so I can't put up his graphs just yet.

After a big more digging I've found out from a mechanic who works for Team Dynamik that some engine management systems do cut fuel completely by means of no injector pulses, where as others tend to pulse the injectors at a low duty cycle. But in EFI cars, all cars have a fuel pump which always runs, a fuel rail, pressure regulator and return line to tank. Fuel is always doing a full loop. The regulator maintains roughly 3 bar on the rail and this is where the injectors get their pressurised fuel from for injection.