As of late, I have noticed that my fuel economy has declined modestly. This is not just the seasonal effect of winter fuel, as I have been looking at it over several months. Interestingly, the INST MPG display seems to lag changes in throttle position / load compared to how quickly it used to respond.
Only modifications are stage 1 turbo mod.
Mileage is about 28K. I'm considering throwing an air filter at it to see if there is any improvement, even though the service interval is 50K. I really don't expect an air cleaner to have a great effect on an engine that runs under closed loop control, particularly one that uses a wideband O2 sensor for the primary Lambda loop.
Also considering cleaning the MAF sensor.

 

Mileage is determined by driving technique provided the car is properly tuned. I'm not sure how instant mpg works in today's cars. In the old days it worked off of engine vacuum. With any engine mods, who ever did the install may have knocked some sensor. Dis the change start showing up after the stage 1 turbo?

If you drive in dusty conditions, the air filter may be dirty. It is an inexpensive fix when just parts swapping. I change mine prior to the 50,000 mile recommended interval.

 

The current breed of MPG indicators work by calculating fuel injector pulse width and VSS data. They're reasonably accurate. I generally do a manual MPG calculation whey I fill up, which is of course, the most accurate method. No dusty conditions that would prematurely clog the air cleaner. I generally drive like granny and get into 4th gear as soon as possible in urban conditions.

 

How much of a decline in mileage are you talking about? The sluggish readings are something I haven't heard of. You might try the old trick of doing the "battery disconnect" and see if it will reboot the DIC.

 

Low 20's in mixed driving. It generally used to hover in the high 20's. Rebooting the ECM is on my short list.

 

could be a boost leak. your stft's and ltft's will be off and bad mileage will result