The computer in your car is a treasure trove of information about gas mileage, engine performance, maintenance issues, and more, but without an OBD2 scanner, most can only communicate with you through the primitive language of dashboard warning lights.
Luckily, two such scanners from Kobra are on sale right now, and both can sync to your phone and work with third party apps to track and record anything and everything about your car. The Bluetooth model ($8 with code CDJHZKQZ and a 20% on-page coupon) will only work with Android, but the Wi-Fi one ($13 with code 2DR4T5CD and a 20% on-page coupon) supports iOS as well.