Are you sure that they do these as part of the Inspection II or did you specifically request the plugs and fuel filter? I stopped doing the inspection I and II services after I realized how little work they actually do.

They used to do winter and summer blends in California. Not that one had any choice in the matter but the "oxygenated" fuels got you poorer mileage with the benefit of cleaner emissions. They have finally phased out the MTBE additive that made such a mess of lakes and such when used in marine applications.
As far as your mileage issue goes, have you actually checked it in the standard way? Sometimes the sensors in the tank will degrade somewhat and the readings can change slightly. Going from "Full" to "3/4" tank in a certain amount of miles is probably not the most scientific way to judge fuel economy. Top off the tank, zero out the trip odometer, and drive the car. Next time you fill up, check the trip odometer and do a little math. I don't know if you ever did that back when the X5 seemed to get better mileage, but it would give you a baseline one way or the other.



Gar

Another thing that could affect fuel economy would be the O2 sensors. At your mileage they are probably on their way out, but will continue to work for some time before throwing a fault. There are four total... I plan on doing mine in the near future.

To the OP and prwdmd, if you do decide to change the 02, only the front 2 are the ones that need replacing, NOT all 4. The front 2 are the ones that feed the DME and change signals, the rear two are there for nothing more than to confirm the presence of cats and to throw a CEL code if they're removed...they do nothing to affect performance or mileage.

I was thinking that the O2 sensors maybe the culprit as well. Maybe I should attack those first and go from there

the air filter will have been changed
I changed my air filter at 50000 miles because it was black with dust and then the engine used more fuel because it was getting more air through the clean filter .
just my 2 cents .

Are you measuring the gas usage and calculating mpg, or just using the gauge?

:iagree:This is where I would start as long as air filter, fuel filter and plugs are new. Also, are you sure you are running the correct air pressure in your tires?

I measured gas consuption reading the gas gauge and kilometers I drove on a full tank to the 3/4 mark.

Just had new snow put on and I double checked the tire pressure with what the numbers on the door so I am ok there. I am thinking O2 sensors are the issue here