I've done the reverse trip to the OP- Chicago to St Louis several times. With the cruise set at just under 80, I used to get 19.5-20 mpg. The most recent round trip with the new Bridgestone Dueler HP's replacing the Michelin Damarais 19", I saw 20.8 mpg with the cruise set at 80 whenever possible.

At the same speeds, my old '97 528 will return 27-28 mpg. But the '10 X5 3.0 loaner I had a couple weeks ago, I was seeing 22-22.5 mpg at the same speeds on a 250 mile round trip to MI and back. Pretty damn impressive for a 2.5 ton AWD SUV!

80mph is definitely the sweet spot for the X5. I find myself cruising at 80mph almost all the time. Gas mileage is awesome. But filling-up is not so fun though. :p: At times I can get 100 miles on a quarter tank on the highway!

Yep, I get about 19-20 (according to the OBC) on the highway. Best bet for me is to set the cruise control. Otherwise, mashing the pedal is too tempting.

i averaged 50mpg this weekend up to NYC and back...in our prius :p

the truck would have been easier to battle the cabs with though.

I wish BMW would bring the 330d over here. You could average 50 mpg's, have the handling of a 3 series and still hit 60 mpg in 6 seconds flat if you get the urge.

Well, in a 330d you could average 41 mpg in the US (49 mpg on Imperial gallons)

What would really make sense to me is the 320d ED, at 69 miles per Imperial gallon (57.4 mpg US) But then North American drivers would have to learn to drive with three pedals again.

Yeah, I mixed up the U.S. mpg for the regular 320d and the 330d. I would give up a little mpg to get the acceleration of the 330d over the 320d personally. The 330d would be the best of both words for me and a nice balance of performance and fuel economy. 40+ mpg is still pretty damn good.