Great question, I have a 4.4i 2001 X5, my buddy did the 0-60 check and we came out with 6.8 also, to be honest it was at 6.789 sec. I'm guessing that the reason why the times are so close together is if you look at the weight on the 01 and any other X5 the 01 weighes a little less then compared to an 04-06... Checked it out with my buddy... So it could be a possibility about the 01 being the fastest 4.4i... Now with the 4.6 or the 4.8, I would have to say the 4.8...but 4.6 not really that far off...