I just finished a quick project that will save me the hassle of doing the trip mileage divided by fill-up gallons on my phone's calculator. To use it, start resetting your trip meter to 0 after a fill-up. Then, the next time you fill up, type in the number from your trip meter as well as the number of gallons you pumped. The app will calculate your MPG and save it. Your previous fill-up MPG will appear to the right, so you can always compare gas mileages between fill-ups.
The app will be free.