It’s hard to imagine an easier way to set up communications between an Android device and an Arduino using Bluetooth than by following this guide. In the center of the breadboard you can see … ...

The Arduino gets the fuel consumption readings from a Microstream OF05ZAT fuel flow sensor. The Arduino processes the data and then transmits it to a smart phone via a Bluetooth module.

The $22 kit includes an Arduino-BLE board and 6-pin header that should be fairly broadly applicable for building Arduino-based hardware that can communicate with iOS devices.