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It seems to depend how your car controls its idle. In my relatively old auto mazda 626 91 year without any modification it would use up the same or a little bit more fuel if i put it into neutral. This was because the idle went up with the reduced engine load.
If the car actively controls the idle RPMs then the car will use less fuel when idling in neutral compared to drive.
The best way you can tell is to use a scan gauge or similar device.
As a general rule of thumb (that i am making up right now) if your idle goes up by more than about 50 rpm its likely your car will use the same or more fuel when idling in neutral compared to drive.
Note: some cars can be put into a "idle tuning" mode where the idle control will be turned off or set to a particular level. If your able to do this you can manually tune the idle with the set screw so that when you turn off the engine idle control, the car will idle as low as you set it. This works only when you can place a switch for this in the engine cabin otherwise the engine might stall with low RPM when in drive at the lights
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