More information at my related blog post.
Updated Sep 2018
This project was a nice idea, but is abandoned now. I seem to recall finding a similar site on the web where fuel mix data was available, some years ago. At that time, I contacted the owner with a view to obtaining the data to help get more eyeballs on it, but he was not keen.
Feel free to create a ticket if you want to discuss any aspect of this, though.