Hey, cool thing! Personal apps are awesome, and I really enjoyed your story.
I hadn't seen you mention OBD in this article, which is built in to most cars from the past 25 years.
If your shared car has it, you could put one of those Bluetooth LE readers in the port, and have copies of your app connect to the BLE signal on "vehicle return" to calculate milegae driven and maybe more, depending on what the car reports!
Might be worth playing around with, either way congrats on your cool thing