I can't tell you what he differences are on your Corsa,
But,
In my wifes Avalon, fully warmed up, with its automatic transmission, according to the
SG;
.33 GPH in neutral, coasting or sitting,
.49 GPH in D. sitting (Thats 50% more fuel)
.00 Gph in D. coasting over 47 MPH (decel fuel cutoff)
1.5 GPH in D. and dropping coasting under 46 MPH
So when I am on the highway I coast in D until we break the threshold of 46 MPH then put it in N until I need more speed again.
My Metro has no High Tech readout, it is too old for
SG, and I don't feel confident enough to build a Guino.
But I have been told by those who have one that the Metro burns ~.25 GPH at idle, (and I cut my idle down to 500 rpm for further savings)
So at 60 MPH that would work out to 240 MPG coasting at idle engine on.
Not as good as EOC, but this is what I do in heavy traffic. (which is most of the time here)
Hope this helps,
Schultz