With my old diesel i noticed a huge difference since they would put anti gelling chemicals in. usually about 5mpg(down from 25-20). With my bimmer, i notice zero different. 23-25mpg 80% highway always using the same mobil 93 octane.
3 stage manifold, Mapleridge headers, AA tune, Injen intake, 135i exhaust fitted to 128 mids, blackline tails, Depo headlights w/ 8000k LED angel eyes and xenons, 8000k xenon fogs, Lowered on eibach springs, m3 lower control arms, 264's in anthracite with a red lip wrapped in hankook v12's