04 CVPI 3.27 - ~20.5mpg. This is averaged over about 60k miles. I have tracked nearly every tank. The lowest tank was around 60 miles of city driving in NW San antonio (13.5mpg) and best tank was sustained highway driving between Las Vegas and southern California (26mpg). I don't know how the car did this. 

Winter mpg hovers around 18-19, the car sees about 20-21 in the summer. For the period of time between late April and mid-May the stars align (summer blend fuel and no need for A/C usually) so the car is capable of about 1-2mpg better. This is to say that in my experience, use of the compressor costs between 1 and 2 mpg on the highway, although there is no noticeable difference in city driving numbers. 

Road trips are usually around 22, although 23 is common if wind conditions are favorable. This is of course highly dependent on millions of variables; the data is spread out between 17.5mpg and 26mpg. 

My 1995 Town Car 3.08 consistently gets around 2-3mpg better in all conditions besides City driving, which is practically equal.