ScanGauge II not exactly an "app" but...

...it is an OBDII reader. I had it first on my Toyota RAV 4 and transferred it to the Prius V when I sold the RAV 4. I did send it back to the factory in Phoenix, AZ for a firmware/software update before putting it in my v so it has the latest sensor feeds. You must put it in "Hybrid" mode to keep it from shutting down every time your engine shuts off. The MPG readout is very close to the dash readout and why it's not the same beats me.

I like this "app" gauge for monitoring my car and being able to find "check engine" codes if they ever pop up. I have a printout of what the code numbers mean in my trunk so I'll know whether to limp home of call AAA for a flatbed to the nearest dealer. The Toyota Prius specialty mechanics like the gauge too B/C they can quickly page through it W/O having to hook up their monitor. If they see something odd then they scan the OBDII with the Toyota monitor.

They are also fascinated at the many handling mods I've put on the Prius v. The younger mechanics are amazed that I have ENKIE RP F1 lightweight wheels on it. They know those wheels on sight. It's fun to see the mechanic call others over to see my latest mod. I'm surprised they are even interested in a Prius but maybe it's just the oddity of it that makes them curious.