I would have thought that the ability of the B58 to get such good MPG is enhanced by the 8 speed auto allowing ultra low revs at motorway speeds.
My N52 powered manual 130i will average 35MPG when cruising at GPS 70mph on long motorway journeys. I think revs are up around 2,750 at that speed.
Yeah that definitely helps. I know on my v6 diesel with the 9 speed auto box that at 70mph with ACC on the engine is barely ticking over, i think its 1500 rpm at that speed.
Old 5 speed or even 6 speed gearboxes count have kept the car at 70mph and revs under 1500.