what is the most fuel efficient twin, per mile flown?
In terms of fuel cost or fuel volume?
A diesel DA42 is pretty comparable with the Tecnam in terms of nm/gal (and slightly faster), but depending on location Jet A may be cheaper
pr gal than avgas.
I seem to recall that you can throttle the diesel DA42 back to some ridiculously low power setting like 40% and get 7.5gph at 120 kts, or something like that.
I really like the Tecnam btw. If only the specs weren't so much on the low side...(620 nm range, cruise 135 kts, ceiling FL150, t/o distance 450m - and a new one is still USD450k...)