Ive been using this app for years and havent encountered issues with lost data; I use L/100km and havent seen the issues reported by other users.
The overall usability of Gas Cubby is quite good and the graph feature is slick. I drive a hybrid and it is interesting to see what a huge difference summer vs winter makes for fuel economy.
There are some user interface issues with some of the tabular views not fitting well into the screen but these are not critical. I think there is plenty of room for improvement, but this app does its basic job well.
Cryolathe about Fuelly: MPG & Service Tracker, v2.7.1