2002 Monte Fuel Milage problem

2002 Monte Carlo SS 3.8L w/58000 Miles - with a bad fuel milage problem. Its getting about 21-22 MPG Highway. It used to get about 28-29. This has been going on for at least 3 monthes. It seems to have less power than when I purchased it. The tach is at about 2200 RPMs at 70MPH. It does shift into Overdrive Fine. I've tried 87-89-93 Octane Fuels with little to no effect. I did have a plugged CAT but has not changed fuel issue. Here what I've had done in the last 2 months, some to do with the problem, others just general maintence.
New Tires - Maitnence
Trans Flush and change - Maitnence
Oil Change - Maitnence
Fuel System Cleaner - Prob. and Mait.
Cleaned K&N Air Filter - Prob. and Mait.
Fuel Filter - Problem
PCV Valve - Problem
Cat Converter - 100% covered by by dealership - Problem
Thank YOU!

Some drop could be expected driving in very cold temps, also have it scanned for proper sensor activity, especially the 02 sensors

Thanks for the advise, I beleive I found it with a simple slight driver side brake dragging. It never pulled to the left or warped the rotor but when I pulled the tire off the caliper side was well worn over the rest of the Brakes. I greased the slide pins on the caliper and replaced front brakes and rotors. Thanks again!