Dekerta Software has a nice app for this, Fuel Economy Calculator. While it's been available for BlackBerry for quite awhile, they have also just released a version for the BlackBerry PlayBook. All you have to do is enter in your odometer reading and how much fuel you purchase, and the app will calculate the efficiency for you. You can track the data over time automatically by averages and a handy graph. This app supports multiple vehicles, metric and imperial units, and you can even keep track of oil changes and maintenance as well as set reminders. Available for most BlackBerry smartphones, you can grab Fuel Economy Calculator for $2.99 for your BlackBerry, or just $.99 for your Playbook.
Contest: Dekerta Software has given us 50 copies of Fuel Economy Calculator for BlackBerry. To enter, leave a comment below. The contest ends this Sunday at midnight PST. One entry per person please, and good luck! *Please note, the contest is not for the PlayBook app, due to App World limitations.*
More information/screenshots and purchase Fuel Economy Calculator for BlackBerry
More information/screenshots and purchase Fuel Economy Calculator for BlackBerry Playbook