2006 325xi Gas Milage Decrease

Hello,

This is my 3rd BMW. About 6 months ago my computer starting registering a decrease in my gas mileage. The car was under CPO so I brought to the attention of dealer even to the the Service Manager. After a few trips they put the car in the shop and did a calibration. Yeah, the mileage I was calculating manually returned to the computer. I usually get 23/24 around town and 28/30 hwy. My happiness only lasted about 6 weeks. Now the car has decreased in mileage manually and on the computer. The computer is off about 2 miles per gallon.
I purchased new tires, had the spark plugs and air filter changed and it is going in for an oil change this week. I even put Techtron fuel injector cleaner in. I am just over 100K. The car seems to be running just fine. I drive over a mountain and drive on a hwy where the speed limit is 70mph. So we have a lot of fun together but I am really worried something is very wrong.

Any idea? The only thing that was suggested was to reboot the computer again by removed the battery cables.

Thanks