Quote:
Originally Posted by Billfitz
I doubt that. At lower speeds vehicle weight is the primary factor with respect to mileage. It's at higher speeds where drag dominates. The official line for having it is: The active rear spoiler optimises the air flow at the rear of the car as your speed increases, to ensure excellent road holding and classic BMW driving dynamics. I'd imagine the reason why it retracts is purely cosmetic. Only the designers know for sure.
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I will somewhat have to disagree (and agree with ipilcher). What ipilcher is saying is that a retracted spoiler will be better for mixed-driving mileage compared to that of a car with a permanently fixed-in-place spoiler. If BMW had a fixed spoiler in place, the quoted mileage of the car would have been lower (not good for marketing). Speeds of 40mph-70mph are not small and are easily reached in day to day driving. I could see the drag of a spoiler affecting the average mileage at those speeds.
Now, at higher speeds, the spoiler comes out just to plant the car slightly better on the road for driving dynamics. Yes, mileage goes down, but then you won't be driving above 70mph if your goal was to extract the best mileage out of your car. Also note that no car manufacturer quotes mileage numbers for cars driven at those speeds.