I use a cool app for my iPhone called Gas Cubby. You can import/export from/to Excel and also backup your data to their servers for easy restore if you want to load it on a new iPhone or iPad or jay want to reset your iPhone and start over. You don't lose the data. It also supports multiple cars. I have all mine tracked. It even graphs mpg, cash spent on repairs, etc.
http://appcubby.com/gas/index.html