Just FYI, if you do a search you'd find the following related thread:
http://986forum.com/forums/general-discussions/30561-rpms-mpg.html
Personally I've been getting a pretty low 20.5, sometimes almost 21 mpg. The only time I get to take long stints on the highway is either if I take a long trip to somewhere else in the state or purposely take a drive just to give the car some movement and miles, and I have a hard time just cruising, heheh. I force myself to use the cruise control as much as possible when taking a long highway trip.
The majority of my driving is typically city, traffic lights and stop signs galore, with short stints on the highway, only 15-20 highway miles at the most when I have my occasional longer commute, and that is not as common as my shorter trip to work and other more local places. I could definitely stand to get even another 5mpg, though I know at least a part of that has to do with my right foot control.