Sorry for the long-ish message, but I would like to find out exactly...

Recently a friend started shifting into Neutral when going downhill, claiming that it would save fuel (due to lower RPM). I tried to tell him that it is totally unnecessary (even if fuel is saved the amount would be negligible), if not using more fuel and causing more wear, however he is not convinced.

Here is my understanding of the Fuel Injection, I think I read it somewhere:
For modern EFI models, when going downhill, with trans in Drive the momentum would keep the engine turning, and thus the computer "switches off" injection to save fuel and actually no fuel is used. If instead putting trans in N and fuel (idle amount) would be required to keep the engine turning, thus in actual fact more fuel would be consumed.

Is the above correct?

How about for older cars with carburetor or mechanical injection (W126)? Does higher RPM translates to more fuel? (Referring to coasting downhill with no throttle applied)

My understanding is that it would based in the workings of the W126 Mechanical CIS.

Thanks for reading and any comments would e appreciated.