Ford Focus getting poor MPG?

I recently purchased a 2002 Ford Focus SE sedan. Everything works perfect on the car and it has been inspected by a mechanic and it is tip top shape and I can tell was well maintained. However, over the first few days I've had the car it seems to be getting poor MPG. I filled it up, drove 100 miles, and it reads half a tank of gas. I think the gas guage is accurate because it was on half a tank when I filled it up and it took approximately 6.5 gallons, which is half a tank in this Focus. So I am averaging about 15 MPG when I should be getting at least 23. What can be causing this if everything else seems to be fine? My driving habits are slow acceleration never over 70 on the HWY, A/C on windows up on the 2 setting out of 4. Air filter is clean, spark plugs and wires are good (what my mechanic said I personally haven't checked them). Maybe this is related or not, Idk, but the radio sometimes shuts off mid-song and then turns back on by itself. It's been doing that since I got it. That's the only problems I've had with the car. What can be causing this???