As said, the PT isn't know for its mileage.
I just started tracking mine (see sig) on July 1st and I'm averaging a little over 21 mpg which is driving to and from work on Interstate 16 at 75 mph and some city traffic.
My most recent fillup was 23 mpg and that was me testing the theory of slowing down, cruise control when possible, etc... etc...
I'm pretty sure I could get 26-27 mpg driving on the Interstate if I had the cruise set at 60... problem is, I don't drive that slow
Edit:
Just saw this today on MSN.... might be worth a read.
Fuel-Saving Secrets - MSN Autos