27/09 - Well, without getting into an extended argument and providing reams of individual engine tests and lengthy scientific examination - Wikipedia provides some basic figures that show a Lycoming O-320 at 0.460 BSFC (29.3% energy efficiency); and a Toyota (Prius) 1NZ-FXE at 0.0370 BSFC (36.4% energy efficiency) - a difference of nearly 25% in efficiency with the Toyota engine.
I don't know how you can expect an engine that has had virtually no engineering development/advances in technology since its inception in the early 1950's, to seriously compete with a modern automobile engine that utilises a wide range of efficiency improvements in its design - such as high-tech fuel injection systems, VVT-i, multiple OHC's, low-friction rings and pistons, high compression ratios, CGI and high-tech alloys in blocks and heads, fractured metal caps for bearings, sintered iron powder for conrods. The Lyco is a dinosaur.
https://en.wikipedia.org/wiki/Brake_...el_consumption