Sounds like a great idea... but... but... but...
The manual data entry overhead is much, much more daunting than you realize. I've done this very thing with several of my vehicles, mostly by keeping gas receipts and batching entries to a spreadsheet, with notations for exception conditions like very cold weather and so on. The amount of time this takes is huge.
I was doing this to track efficiency and cost of operation. What I discovered is that nearly everything I needed to know was documented in the first 10K of ownership, anything further was simply confirming the trend to two decimal places. The most eye-opening use of the information in one case was discovering that with two otherwise identical cars, one a MT, the other a hybrid, cost of operation of the hybrid was higher. Ain't that a kick in the head?
Anyway, there are a boatload of phone apps that do this for you, mostly for business expense documentation. Just find one that hits most of the points you need and tweak with manual notes whenever needed.