There was a 2% mechanical efficiency penalty for overdive, compared to 1:1, with normal mineral gear lube.
Since the difference was in hydrodynamic churning losses in the extra gear set, that relationship is no doubt less now, with contemporary all-synthetic, lower viscosity lubrication packages.
As of 2006, the National Academies of Science published that ICE vehicle powertrains absorbed 5% of transmitted power.
A 1:1 transmission would knock that down to 4.9%.
28.1% of available power would be available at the traction wheels, instead of 28%. A 0.0035% gain.
It's my opinion that, in a capitalist nation, you wouldn't want to even waste the time thinking about the difference it would make, let alone act on it.
