Wait, you have a x5 30D? This would be a different engine compared to the US version.

Please tell us how many gallons of diesel you fill up with to achieve the ~260 miles. Something doesn't sound right about your mpg, but perhaps you like to fill up at the halfway mark?

I used to fill 70 litres (UK) which lasts 280 miles (The reserve light comes on), this is now only 260 miles. This used to be 18.2 miles per UK gallon and is now 15 miles per UK gallon.

Not that I think that 260 miles/tank is expected, but what are your driving habits? I drive many short trips, which is not quite what a diesel engine does best with. I would get quite a bit less than most report on this forum out of my 35d: 350-420 miles/tank. I would usually get about 375. Obviously that is still quite a bit more than your current 260.

I would definitely question the dealer why your mileage is so sucky.