If one wants range and efficiency in an "old" design, then look no further than to the Piper Aerostar. Yes, it's known for being a fast rocket (the fastest, in fact), but that also means it's the most efficient. The 700 will do 260kts on 45-50gph. At 205kts, it'll sip 25-30gph. Go down to fast single engine speeds, around 140-150kts, and the Aerostar will probably not drink much more than 15-20 gph (a guess). That's rather impressive. With a standard 165gal, or optional 210gal tank, it's easy to see just how far one could go.
To get efficiency there is only one thing you can do - reduce drag. Aerostars have one of the narrowest cross sections and a very high wing loading, so that's why the have minimal drag.
