I have a 2000 GS and Get roughly 17-17.5 in the city. Highway varies on speed, around 30 if cruising gently at 65 and about 26-27 at 75-80mph.
I had an old 97 that got about 14mpg in the city and I had vacuum leaks all over the place wreaking havoc. I'm not sure if routing is any different on a GP than a Regal but the entire kit at the dealer is around $50 or so. That instantly cured the problem, went to 17+ all the time.