When support goes, so do users
In its original form, Gas Cubby was a very simple, elegant, and useful app. However, its purchase by Fuelly was followed by months to years of neglect. Loss of the sync function was particularly troublesome.
The apps new owners have repeatedly promised a comprehensive reworking. That still may happen. But not keeping up with new iOS versions over several years has said to many users that Fuelly lacks either the interest or the ability to keep this a viable app.
While Road Trip is more expensive and slightly less intuitive, the RT interface and sync functions long ago left Gas Cubby in the dust. I have not looked back since switching.
basic iPhone user about
Fuelly: MPG & Service Tracker, v2.7.5