Hypermile is a Java Android mobile app that provides vehicle economy metrics. It requires the use of an ELM327 Bluetooth OBD-II scanner to retrieve real-time data from the vehicle ECU.
This was a final year university project which I received a high 1st.
To be able to develop this app away from a vehicle required the use of the ECU Simulator, another creation of mine. Even when developing mobile apps I can't help but get microcontrollers involved somehow ;p