About 12 mpg iirc, e39 m5 gets about 17-19 mpg average, e60 is far worse on fuel and also has a shitty stupidly small fuel tank as well, just for added annoyance to the already crap mpg figures.

M3 e9X isn't too great on fuel either, you'd be looking at 20mpg if you're only modestly using it, or less.