I have a 1990 Audi V8 Quattro with around 100K miles on it (I know, I know, you can keep the jokes to yourselves). For a big V8 with all wheel drive it's always been pretty good on gas, around 19 - 21 mpg on the highway. Lately however, it has decided to not be so good on gas, and I'm lucky to get 13 or 14 mpg on the same commute, same driving conditions, same (or even more conservative now) speeds. I think this sucks, because gas ain't gettin any cheaper. The weird thing is that the car runs perfectly - smooth as silk, tons of power, no smoke or smells. I would think it would run like crap and smell funny if it had sensor issues and was running rich. Anyone have any ideas ?
Tires are new and all are set at proper pressure

I have the same car, I too was having this issue, I did a nice solid tune up, IE.. Wires, Distributor caps, and spark plugs and air filter, also you need to run good gas none of the citgo brand gasses, only shell or BP trust me it makes a difference.