[mpg_numbers_guy]

Fuelly indicates 25-30 mpg average.

My Civic was rated at 30 (29-34 real world according to Fuelly) and the max I got was 60 mpg on the highway at 55 mph with a slight tailwind. P&G was 50s around town.

IMO you'll need a boattail and a manual swap (since the older automatics will be dreadfully inefficient) in order to do engine off coasting, unless you swapped in something like the Insight's 1.0L drivetrain and manual transmission.

I personally don't see the possibility of doing it but look forward to see what you do with it. Maybe I'm wrong, I'd love to be wrong in this case! Your largest enemy besides drag is probably the automatic transmission.