I can only guess what all it involves...but I'd have to say grooves...lynz...head, valve, and intake work...modified sensors and an EFIE...ignition upgrade...new O2(s)...PCV jar...oil additives eng/trans/diff....etc.
It's probably a case of putting it all together in a way where everything done contributes to mpg gain...and nothing done detracts. And tweaking it all to max out mpg.
Sort of discouraging for bumblers like myself who've managed +35% gains with a couple of cars? But unless you have the right feedback..scangauge...gas analyzer..etc...and the knowledge to use them...and make & test the mods....??
Leading the perpetually ignorant and uninformed into the light of scientific knowledge. Did I really say that?
a new policy....I intend to ignore the nescient...a waste of time and energy.