Since the X6 is heavier (according to one BMW website), and since the X6 is reported to have a higher coefficient of drag (assuming the same frontal area), then if they had the same engine the X6 wouldn't outperform the X5 at all, it wouldn't keep up. Depending on the suspension differences it may handle better (or worse). It is all theoretical at this point, since the published figures for weight and drag are for the X5 4.8i

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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