I stick by my original post. It seems like gas costs are very important to you, so considering switching to an X6 Xdrive 50 is not going to be an upgrade at all in gas mileage, but it would be a performance upgrade. The X6 Xdrive 35 would be a slight increase in MPG (maybe $600 per year or so depending on how much you drive), but probably a wash performance-wise with your 4.8i. If fuel costs are such a big factor, I really don't see why these are the two models you are considering. Also, if you do 80% of your driving in city/stop and go traffic, how many 90 mph downshifts are you getting?

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Profeshenal spellar