Pretty good.

I have broken 30 mpg US, 37 mpg Imperial, with the 535i. That is with steady highway cruising in the mountains, two people plus luggage, 92 pump octane, and not taking any particular advantage of the 300+ hp. I think both figures are good for reasonably sized, performance-oriented vehicles (but I get to shift a real transmission, lol)

And people think they need diesels. Bring on twin turbos, direct injection, and intelligent design, and it matters a whole lot less whether the engine is using the Otto cycle or the compression-ignition cycle.

edit: fixed typo

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2008 535, 6 MT, M Sport, Premium, Space Grey
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue