Unless your goal is top speed, going to a 3.15 ratio is going to decrease 0-60-100 time. You might get better mileage cruising at 2200 RPMs if that is your goal. Isn't there a 5-8 gear GM transmission that would be better for both acceleration and cruising?

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas