As others have said, try the following:
Reset odometer to 0
reset average MPG
Now, start your drive from when you are home. Your initial numbers will seem low, perhaps 21mpg for the first part of your drive, since you will be going from 0 miles per hour and hitting stop signs, traffic lights, etc. Once you get to the point where you start to drive in better than stop and go traffic, your average will start to go up. Since you will be burning fuel while backing out of a driveway, or stopped, that causes your average miles per gallon to seem really really low initially, but the average will go up as you get to really drive.
As I said, I have been seeing between 30 and 34 miles per gallon as my daily average, which I do the above resets on the trip computer for. The 30 as my low was a bad traffic day with above average time spent in stop and go traffic. This is a combination of highway and "local roads" driving, and I have been using the "cheap gas", 87 octane no-name stuff rather than going out of my way for better gas from Mobil, or Shell. I am sure that if I put that better brand of gas in, that it may result in an extra 1-2 miles per gallon, but I need to drive an extra few miles out of my way to get there, and will pay an extra 10 cents per gallon for regular 87 octane gas. Would the trade-off really be worth it? Probably not.