That's interesting, I recall that the official BTU content of gasoline has been defined down, without respect to additives like MTBE or Ethanol. If you read older primary source documents from say, the 1950s or 1960s it seems to me it was listed as ~ 120,000 BTU per US gallon. Today, it's something like 111,000 BTU. What happened?

I thought higher octane fuels would return a bit more mileage if the ignition timing was advanced to take advantage of it. One thing I've never quite been able to understand, if the air fuel mixture is leaned out too far, efficiency or fuel mileage actually goes down.