I think the reason for getting especially good MPG when at low RPM is largely due to the slow burning mixture at low power settings.
For example, at FL100, wide open throttle i.e. about 21" MP, 2200rpm, peak EGT / LOP, I get the best range of about 1350nm to zero fuel. This remains more or less constant up to FL160 or so, after which it gets worse again because one needs more power i.e. 100F ROP and more revs to get more air into the engine.
Whereas a turbo engine is never short of air and thus never runs at these low power settings like 16-21" MP.
I think this explains why TB21 owners get only about 80% of my range. Of course they do get there quicker
