How did I get 15.5 MPG?

[Poseur]

Another opinion to back up Orange and Phil. Lower octane gas actually combusts more violently than higher octane. It is less resistent to ignition and therefore fires off faster then high octane. On a modern, computer controlled vehicle you might see an increase in performance with higher octane because the computer will allow the timing to advance due to the increased resistance to detonation. If the engine isn't exceptonally prone to detonation with a lower octane fuel,(ie. lower compression) than this benefit is only imagined. In an engine such as a rotary which has a much lower tendency for detonation you actually see increased performance by using a lower octane.83 octane was the choice of racers for years until the sanctioning bodies made it illegal to level the field. Forced induction changes these rules and in that instance you need the higher octane to prevent uncontrolled detonation and engine damage.
At 40,000 miles our 04' has an overall average of 11.5 mpg. I always use 87 octane and have had no issues with detonation. I can average 13-14 on the highway at 70-75 but as soon as there is any stop and go I'm back down to 11.5. On a trip to Savannah where we drove kinda fast 80-90 we averaged about 10. Seems this is about normal for the H2.