MacAppStuff Trip is a free app that lets you calculate trip costs. There are four different types of calculations that you can make with this app, and each can be accessed via the buttons located at the top of the screen. The first mode lets you calculate the cost of your journey. The app can do this by using some data that you have to input: Cost per litre, MPG (miles per gallon), and distance in miles. When you enter all the data, the app will estimate the cost for you. When I tested this app, I wasn't able to get an accurate calculation. For example, I told the app that each litre was $5, that my car got 7 miles per gallon and that the trip was going to be 100 miles. The answer that I got was that the trip was going to cost me $2.71 and that I would "get" 14.29 gallons of fuel. That is absolutely wrong. I can't go 100 miles with a little over half a gallon. Also, why mix litre there with gallon and miles?
The second mode lets you calculate the MPG usage of your car by inputting how much a litre costs, how much you spent, and the distance driven. The third mode lets you calculate how much fuel you are going to use to drive an X number of miles and the last mode can estimate how many gallons you get for a given cost.
This app is a good idea. It could make things easier for you. However, I find it hard to trust it after the wrong calculations that it made for me.