I get about 19mpg, have to fuel up at least once every other week driving 30mi round trip (~300 for two weeks) to work at 65-70mph (well, about 5-10mi of that is 35-40mph). I'm absolutely certain I'd double my gas mileage. lol
Edit: oh, you probably meant over a new non-hybrid. Yeah, it might not be much better, but I do drive around the back-roads some times so it'd be nice for that.
My commute is only 6.5 miles each way, but it's 100% busy surface streets, during rush hour, with long red lights.
My previous car (2002 Camry 4 cyl) averaged 18 MPG.
My current car (2013 Camry Hybrid) averages 35 MPG, so almost twice the mileage.
The reason I only get 35 MPG is due to the short commute.
It takes at least 2 miles for the car to warm up before it starts operating in full hybrid mode.
If my commute was twice as far, I'd easily be getting 40 MPG.