On the non-complaint side...
My first PT was a 2006 na base model; with standard shift. I put a AirRaid intake system on it to open up the breathing; and I must admit I liked the growl so much I undid the gas mileage advantage.
But, about a year after I owned the soonercruiser, I made a long trip out East, downwind, spring weather where no ac was used, and got a smidge under 30 mpg on the way out.
Coming back west/home was a different story. It was into the face of the wind, and lots of rain. Only got 25 mpg.
Of my turbos, the 2006 GT convertible, my Black Sheep, got the best mileage. On another long trip to Florida and back, I got a combined 25.5 mpg. I thought that was pretty darn reasonable.....for a brick that she is. Around the city, I'm sure my turbos are only around 20mpg.
Thank GOD for the low premium gas prices.
Be sure to keep good records, and calculate off of many fill-ups.
OH! One more thing reference MPG!
I only use 100% gasoline! Otherwise, you are losing 1-2 mpg.