If you're concerned with gas mileage the E9X M3 may not be a good idea. I have managed to get around 19.1MPG according to the computer (not the most accurate measurement). My car is not my daily driver and most of my driving is cruising the back roads or longer trips on the Autobahn.
Normally, I get about 220 miles to the tank with city driving and 310 with highway driving. On the highway, once again according to the computer, I see 21mpg traveling at a steady 75 mph.
2011 Le Mans Blue M3// Black extended Novillo w/ Blue Grey Aluminum trim/DCT/ZTP/ZCW/ZPP/ZCP Premium Sound ///Akrapovic Evolution Exhaust
2015 Audi S4 Prestige, Brilliant Black S-tronic
2012 Le Mans Blue 135i M Pkg DCT