Carculator can calculate the cost of fuel for a journey before you drive it, or after you have driven it!
You can calculate a journey before you drive it if you know the distance of your trip, by selecting Calculate a Drive.
If you do not know the distance and or would rather view statistics of your drive, such as average speed, times at various speeds and distance at various speeds then select New Drive. From there, you can start, pause, resume and finally stop your journey using the appropriate buttons. Carculator works in the background, so if you switch over to your Music or Maps app to get directions, then all of the statistics will be working away in the background for you, ready for when you have finished your drive.