This is just an anecdotal observation, but I have seen threads here and elsewhere that 2010 x35d are getting 30+ mpg on highway trips. I have 17K miles on my 2009 x35d and have at best seen 27.6mpg. I have taken some 250 to 500 mile trips recently.

I am not complaining, after 9 years driving a Lincoln Navigator.

I usually drive 75 mph on highway trips, so that may be a factor compared to slower drivers. I use the cruise control constantly. Other than that, my X5 is well maintained, I use Gulf diesel typically and run the stock 18" tires at 32 psi.

I have read that the ECU remapping of diesel engines can improve horsepower and mileage. I wonder if the 2010 has been remapped or otherwise optimized for better mileage.