Your car never drives without the choke on is what we used to say.
Now I guess it is the cold-start enrichment on. You need about 5 miles before things warm up and you get to normal air/fuel mixture. Notice your temp gauge when you get to work...bet it doesn't read a full 180.
I dont have an "S", but my standard 2000 Boxster gets in the high teens when playing, and 30 mpg cruising on the highway on long road trips. Normal driving mid to low 20's.