Arduino as Bluetooth Sensor for Runtastic on Android device

Good day,
I hope anyone can help me with this. I'm making a speedometer for my bike, and as the idea gets mature I thought about making it talk to an android app so I can merge the gps, internet connection and other great things on android devices with the data read by the arduino. At first I was going to make the app all on my own, since I know how to communicate them by Bluetooth, but then I realized that using an already mature environment like Runtastic and adding my arduino device as a Bluetooth sensor is a better approach.

The thing is that I've been googling around and it seems that nobody has done this before, so I'm here to ask how does the app receive the data from Bluetooth. I don't know the protocol nor the specs or if is there an API to do such thing.
So I would be really thankful if somebody guides me.

Thanks beforehand and I hope that this question will get helpful to other people.

Edit: The problem is not making the speedo, nor communicating arduino and android via Bluetooth, the problem is how to integrate the arduino side with the cloud, wind, gps and calories counter of apps like Runtastic Road Bike.
They have a menu option for integrating Bluetooth sensors, but I don't know how to make my arduino Bluetooth device to show there.

I'm making a speedometer for my bike..........The thing is that I've been googling around and it seems that nobody has done this before

I don't think you have been looking very hard. There must be a bazillion Android apps out there that that will do what you want. Indeed, any app that uses GPS should give you a speedo.

No, I think you did not understood what I meant. I know there are apps that do that, but what I want to do is to use an app like runtastic for the gps-map integration and add the arduino for a precise cadence and wheel revolutions per minute sensor.

They sell them but they're expensive and I already have the hardware to do it myself.