Looking for (free Android) mileage app, or might write my own

Soon I will be replacing my Gen2 (2006) with a Gen4 (2019), and so I need to start tracking my mileage over again.

I have paper records going back to my 1985 Camry, but now it's time to track my fuel purchases by using a mobile device.

I am looking for a (free) Android app for fuel tracking, but I suspect that at some point I will write my own. I am an amateur Android developer with lots of free time.

Are there (free Android) apps that people like?

Elsewhere on PriusChat, I read about Fuelly, and I just installed it. It seems OK, but I think at some point I would want to write my own app, because I want (a) simpler data entry, and (b) data summaries/reports of my own choosing (like, maybe, comparing mileage with the same month in previous years?).

If I write my own app, I need to plan for what data to record. In the past, I recorded odometer, date, amount of fuel, and amount of money spent. To me it seems pointless to record the price per gallon (because you can compute that, approximately but close enough, from fuel amount and money amount). I might record the location (latitude/longitude) where the fuel was purchased, because that is trivial to do automatically.

In my paper records for the 2006 Prius, I also recorded the mileage as indicated on the car's display, because this has been the simplest way to track the (declining) fuel efficiency over time (especially because I never computed the actual fuel efficiency). This was especially appropriate on the Gen2, because the car reset its MPG display automatically every time you filled up the tank.

I imagine that the Gen4 displays more data then the Gen2, and I don't know whether it automatically resets upon fill-up. So I don't know whether it still makes sense to record the MPG from the Prius display. If I can easily compute the actual MPG, the only reason to record what the Prius displays is so that I can evaluate the accuracy of what the Prius is telling me.

I have other ideas for features that I might want in my own app, but those can be added later. What I want to determine now is what data to record.

Any other thoughts on tracking mileage?