Your driving patterns, habits and the environment you drive in are HUGE. I can't overstate how important that is.
All of the MPG numbers given below are from the computer, which tends to read 1 - 2 MPG lower than calculated MPG (miles driven/gallons used)
My daily commute is 1.2 miles each way, and there are 4 stoplights in that distance, most of the time I get them red. Still, it takes me roughly 5 min. to drive in to work, so the engine never warms up at all. So, my daily commute mileage without other trips is roughly 11 or 12. This makes sense, the engine runs rich when cold, and it's all stop/go driving.
The area where I live is a mall-infested suburb, and is a traffic nightmare. In addition, it's a VERY competitive environment, caused by the highway dept. decision to sync the numerous stoplights so that if you aren't quick off the line and aggressive as hell to boot, you're going to catch every stinking stoplight. I think they expect that maybe if you're caught by a light in front of some mall or store, you'll stop and buy something. Idiots. Whatever. In any case, around here, you either have your foot in it heavy most of the time, or you stop 6 or more times per mile for stoplights.
Driving in this area, like a Saturday spend running errands, shopping, etc., nets me about 13 - 14 MPG.
Driving around in the outlying county roads, where you have occasional stops, cruise speed around 60 will result in 23 - 24 MPG,
A 180 mi. trip to a nearby city on a limited access superhighway, with cruise speeds of 70 - 75 gets about 20 MPG
As you can see, the biggest effect is how you have to drive it.