When I test drove a 535i (after I purchased the damned 4.8i), I too noticed how much better the 5er transmission was. Gear shifts were faster, more precise and when going up steep hills/inclines, the 5er did not aggressively downshift & hold the gear until 4500rpm. BMW must have just thought that X5 owners wanted a "truck" like feel.

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2008 X5 4.8i
Alpine White, Black Nevada Leather, Bamboo trim
Premium Pkg
Cold Weather Pkg
Technology Pkg
Sport Pkg
Rear Climate Pkg
Satellite Radio
iPod Adapter
Automatic Liftgate Operation
20 in Y-Spoke wheels