This app was mentioned in 8 comments, with an average of 1.13 upvotes
If you're not against using an app, I find FuelLog really easy to use. It's super quick to enter everything in when I'm at the pump.
Then I can look back at the log of gas purchases, see how much I spent, what my fuel economy had been from tank to tank. Once you get enough data, you can use the app to calculate how much fuel you need to go X miles and then could budget for that. You could transfer all that info over to your journal when you had time.
In its simplest form, you would need columns for odometer, gallons, price per gallon, total cost, and MPG. Then record columns 1, 2, 3, and 4 each time you fill up. You can math out the MPG in column 5 later if in a hurry.
Price per gallon would technically be optional, but I think it's good to have to see gas price trends.
Ah, a third source. All three are different in my '16 hybrid. When I drive, I tend to run about 45 in the running history. My wife gets just below 40. Actual is about 2mpg less than that. If your readings are saying you're 10 below that, I recommend doing a log to see what you're actually getting. Might be a sensor that's off. Something like http://www.freemileagelogs.com/free_printable_mileage_log.php or https://play.google.com/store/apps/details?id=ch.simonmorgenthaler.fuellog will do the trick. It'll take you two tank top offs to get an initial reading.
If there's any other weird sounds or vibrations when you're driving, get it to a mechanic, though.
Your plan for the vehicle covers all maintenance for 4 years? That's pretty exceptional. If you're dipping into investments to buy it that's a bit more risky but it sounds like you've made a choice. I made a similar choice to yours and I'm quite happy with it but there are times when I wish I had a bit extra to put away instead.
Edit: if you do buy it, I recommend an app like this where you can track the various costs of car ownership and operation: https://play.google.com/store/apps/details?id=ch.simonmorgenthaler.fuellog
FuelLog: https://play.google.com/store/apps/details?id=ch.simonmorgenthaler.fuellog
Track your distance, fuel costs, maintenance costs, etc. Gives a bunch of useful stats about your vehicle like cost per mile, mpg, etc.
For your second request, are you just looking for an app to track mileage? There are several apps where every time you refill your tank you can enter in your odometer, how much you paid, and how many gallons you got. Then it will give you data and some give you graphs and such to track.
EDIT: Here's one I've seen someone else use which is nice, FuelLog And another that I just googled. It's all material design and such. Fuelio
FuelLog - Car Management (GOOGLE PLAY)
https://play.google.com/store/apps/details?id=ch.simonmorgenthaler.fuellog&hl=en
Or get a fuel logging app, I use fuel log
iPhones are better anyway ;)
Anyhow, try this: https://play.google.com/store/apps/details?id=ch.simonmorgenthaler.fuellog&feature=search_result#?t=W251bGwsMSwxLDEsImNoLnNpbW9ubW9yZ2VudGhhbGVyLmZ1ZWxsb2ciXQ..