I agree with Lola 100%. How often you fill up when you don't know how far you are going actually has nothing to do with mpg.

But 7 years of ownership with a consistent steady driving comes to about 140 miles per week. If you are sitting in traffic, driving only in city, and re letting it warm up through winter, you might be getting a little less mpg than you should. But if you go on vacation even once a year for a week, you would be getting over 20mpg. which with heave city only driving is about normal.

I am also betting you have an automatic, so you probably are not locking the convertor in each gear based on the speed you are driving, there are threads about how this robs fuel.