2006
The X5 was not fitted with the new BMW R6 engine, which increased power for both the 2.5i and 3.0i I6 versions of the 2006 X3 and Z4 significantly. It is rumoured that BMW held back deploying the engine in the 2006 X5 so as not to waste development costs in retooling the assembly line, and to make way for a more dramatic unveiling of the all new 2007 BMW X5. In fact, 2006 also marks the end of the E53 X5 as well.
Read more about this topic: BMW X5 (E53)