Sounds as if there's a problem.
I get a good 30 mpg on my mixed drive, and even on a trip where I'm gunning her around the B roads like a madman, the journey consumption is around 24-25 mpg. I record my mileage and consumption against actual fill-ups and never dropped below 26 over a full tank. (Note, this is for a Brera 1750 TBi but I can't imagine it would be much different). Consumption has improved over the 15,000 miles, but only by a couple of mpg.
If it continues at 17mpg, I'd have her checked.
I believe what doesn't kill you makes you...stranger.