Good info in some of the posts above. First thing is the gasoline. If you were using a non ethanol brand and have now started using an ethanol mix your economy can suffer as much as 2-3 mpg as the ECM will adjust the fuel delivered based ont he information from the MAF, MAP, and upstream O2's.
The other thing is if you have not been running a catchcan that catches the majority of the oil that is ingested into the intake air charge, you probably have substantial carbon/guk buildup on the valves, piston tops, and combustion chamber that will result in loss of power & fuel economy. The heat dosent help with the power either.
To check to see if this is the case use a boroscope to look into the cylinder through a spark plug hole while the intake valve is open and you can see the build up.
I suspect you have a combination of all the above. The proper oil separating catchcan (only a few actually catch all or nearly all the oil, but any is better than none) will prevent the build up and pay for itself many times over.
