From what I read on the forum 80MPH is ~3125RPM, and I can attest that's correct.
Also 75MPH is 3k RPM.
I know, because I do this on the freeway regularly.
I get 33MPG avg, not sure if it's because I'm doing longer stretches of 80MPH, or because the engine is still brand new?
MPG goes up significantly, if I go slower.
I find this engine runs best at 2500-2750RPM for highway cruising, which is 65-70MPH.
Quiet, good MPG, smooth-no-engine-vibration riding.
At 3k RPM at constant speed, there's still no Turbo boost, but when I press the pedal it does generate boost.
Unlike my Chevy Cruze, the AC on the ST is not only colder, but it also doesn't seem to affect MPG readout.
Not sure if the AC really doesn't strain the engine, or if the on board ECU just doesn't take the extra load into account.
The ST's engine is 200cc larger, the cabin is smaller (easier cooling), and the gearing ratio is actually smaller than the Cruze, which should help with less MPG drain (it was about 10% of MPG lower on the Cruze in final gear with AC on, vs off); however, when I'm at a stop light, my HUD plugged in the OBD2 port, does show a difference in MPG. (5l/hr with AC, 4l/hr without ac).
I also set my AC between 75-78, so it's not full on cold, and fan is at setting 1-2.