Assuming constant engine efficiency versus power, the best MPG is achieved at Vbg (best glide speed).
This is however far too slow for most people... (95kt or so for a TB20).
Also, engine efficiency is not constant. As the power output is reduced (and at Vbg most types will be running at maybe 40% power) the pumping losses become increasingly significant.
So the best MPG speed will end up being somewhere above Vbg.
In practice (NON turbo context) one flies by choosing a power setting which is good for engine life, say 65%, and sets up that power setting at peak EGT or slightly lean of peak, and accepts whatever speed one gets.