Fuelly will report a fill-up to fill-up change as an amount of mpgs or L/100km or whatever. You can also add the vehicles fuel economy ratings to its entry, and see the difference between your average. I recently suggested that should be in percentage.
As to the octane, I think just tracking in the notes per fill-up is enough. It is what I do, and only a small amount of cars will show a difference. Then a smaller amount of people will experiment with it.