If you do a search, there are numerous articles about the subject you are talking about. The difference between a 4.6 and 4.8 price wise is only a few thousand difference. While the 4.6 does have less MPG, it is very minute. We are talking about an extra $100 maybe $200/ year depending how heavy your foot is. If that few hundred is compelling you to go one way or the other, then don't get a X5. But since you have an M3, you know of all the goodies that come with it.

I just did a search a 298 articles came forward, 6 of them being direct; "4.6is vs 4.8is"

Hope that helps and good luck with your search.

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2011 E70 50i M Sport