I've made several mobile apps (using a few different tech stacks), some desktop apps (mostly for Windows), and lots and lots of web apps.
A few questions I have:
- What kind of "App" are you talking about? Is this something that you need to use on your phone? App is really an ambiguous term nowadays, but I assume you mean mobile app.
- Is this something that only needs to live on your phone, or are is this something that you want available to other people?
- When you say "track my mileage", are you expecting the app to track your miles driven automatically or are you expecting to enter some data manually? At the very least, you'll have to enter the gallons of gas that you buy?
- Are the owner of the website that you want to upload data to?
- If so, how do you plan to upload the data?
- If not, does it already have a mechanism that supports this (usually this is called an API)?
To answer your question:
Creating mobile apps kinda sucks, especially if you own an iPhone. If you have no development experience, you won't find it to be easy. If you want to incorporate location data, it gets even harder. There are just lots and lots of "gotchas" that you get a long way. Products like Zenroad that Sic linked above make it a lot easier, but it's not the kind of thing you can expect to turn out in a weekend if you have no experience.
I have to imagine there are already apps on the marketplace that do this kind of thing. If not, the easiest way to approach this is to get something that lets you store/calculate arbitrary information. If you get something like that, each time you get gas you can simply enter your car's current mileage and the number of gallons of gas you just added. At that point, some simple math produces your MPG since your last fillup. That math could be done anywhere and then uploaded to this website.
A few years ago, I used an automation app like
this to create a little automation for calculating group lunch costs at a place I used to work at. I could enter in a few pieces of data, take a picture of the receipt, and it would calculate the cost per person and email the people that attended with how much they owe. I can't find the exact app that I used (it wasn't this one), but I expect you could do something similar with this. That would be much, much easier than creating your own app.
EDIT:
I found the app that I used before:
Shortcuts. Since I last used it, it was bought by Apple and renamed. If you're on Andoird, I'm sure there's something similar.