Why Is My Volvo S60 Getting Such Bad MPG?

it has a 2.5 turbo engine in it. It says to put high octane (premium or above) gasoline, due to the price I occionally put that in but most of the time it gets regular. Im only getting 17.6 MPG though. We live in the mountains, so there are lots of curves and mostly all brake and go. Is it normal?