I mean FADEC engine is more fuel efficient than those engine which require manually adjust mixture. This is only a general point, cause there are more than one engine in each category, and the efficiency of an engine depend on many aspect of an engine, and it certainly depend on the Engine Control Unit algorithm in the FADEC engine.
I remember I saw a graph that compare fuel efficiency of rotax 915 is (A FEDEC engine) and those older types, like lycoming 0-325 etc. But I can't find it now, although it is more like a claim than evidence.
I believe my claim on the fuel efficiency is valid, just think there is no reason a properly programmed Engine Control Unit couldn't outperform a man, cause the computer monitor and adjust the parameter of the engine multiple times per second, a man at best level, one time in several minutes. (Actually, some pilot claim they ignore the mixture setting when they shouldn't)