Since we are discussing X5M-like mileage, we should clarify that most X5M drivers don't drive with the accelerator floored all the time. They use the power when they want it, probably a very low % of the time in most cases. Just like you likely would do with the tune.

The EPA figures are 2 mpg difference between the X5M and the 50i. More than that in practice would be due to more than occasional use of the available power. The 2 mpg difference is on a dyno, with the same acceleration profile, so that is the approximate penalty for having the power available for when you want it. Any lower in practice is due to actually using it.

Hope that helps.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue