My fuel milage has increased from around 24 highway to now it's 27+.I keep my car in tune and keep an eye out for the small things that can steal power and fuel mileage.
Leaking Vacuum hose's ,let more air into the engine,amm has sent a siginal to the computor for how much air is going thru ,throttle position switch adds to this,now with more air being admitted to the engine(because of a vacuum leak) than read by these two sensors, the last sensor in the chain (02sensor)reads that there is more air than was originally metered fuel for,02 sensor sends signial for more fuel that the car needs, your running rich now and have lost power and mileage.
I may have read this wrong but this is the way I think it happens.
Hope that helps
Poolman