I'm looking for ways to track real-time fuel efficiency data while driving, using an app on my phone. Will a Bluetooth OBD adapter generally allow me to do this?
Yes the OBD-II port can supply data about average fuel economy and instantaneous fuel economy. There are devices on the market that can be purchased and used for fuel consumption monitoring as well as data delivery on a number of different processes going on in your vehicle.
We're advised to not suggest specific products on this forum as they are very opinion-based. However, a quick google search can provide you with plenty of options to explore. I'm not sure if you already had an app in mind that you wanted to use, but I assume that google can shed some light on that topic as well. Good luck!