Alexd1632. If you were running 19" wheels and recently changed to 22" (which are likely EVEN wider), that would have an immediate impact on your mileage as you now have more rolling resistance. Add this to the fact that you likely enjoy the car even more and may be putting your foot even more often on the right pedal. Conversely if you put 17" wheels on the car, your fuel consumption will decrease.

I had a similar experience with my former E34 touring (525i 24V engine). I converted it from automatic to 5 speed manual and gained nearly 2 MPG (US figures). When I would remove my winter 15" wheels (6.5 width) and install my 17" summer wheels (8" wide), my mileage would suffer (I'd lose 1 to 2 MPG).

As Terminator pointed out, put your car's details in your signature line (go to the ID & set-up part of the forum). Then learn the maintenance schedule for your car and see when YOUR car last had these done. If you don't know, do it.

You should change:

engine oil & filter
air filter
microfilter (also known as the pollen filter)
fuel filter
change oil in the transfer case, transmission, front & rear differentials
spark plugs (if you have a gasoline engine)
coolant (BMW genuine coolant only)
brake fluid (every 2 years, DOT4 suitable for an ABS equipped car)
look at updates to correct endemic design issues of the E53, some of which are dependent on the engine & transmission of your car.

Then you can see if you have any thrust rod bushing, rear suspension links or other normal and typical E53 wear items to attend to.

__________________
2002 Euro X5 (E53) 3.0 diesel (M57)
5 speed manual with UUC short shifter.
Pre-xDrive NV125 transfer case (chain & gears).
Factory Xenon headlights, Spyder LED tail lights
Updated vortex crankcase breather.
Motorsport dead pedal, factory fire extinguisher, factory first aid kit, factory F&R mud flaps, factory PDC

Bilstein B6 shocks F&R
Redline oil has replaced "lifetime" fluids (F&R diff, manual transmission, transfer case and P/S).
Dimple magnetic drain plugs in all compartments.