Octane Levels
The most common levels of octane grade are 87 (regular), 89 (mid-grade) and 91-93 (premium). The octane number of the gasolines offered for sale are set by gasoline refiners and marketers based on their perception of the technical and competitive needs in the market. Federal and most state regulations do not regulate minimum octane values, only that the postings on the dispensers accurately reflect the octane number of the gasolines being sold.
Gasoline with a higher heating value (energy content) provides better fuel economy. Traditionally, premium gasoline has had a slightly higher heating value than regular, and, thus, provides slightly better fuel economy, but it is difficult to detect in normal driving. There can be even larger differences in heating value between batches of gasoline from the same refinery, between summer and winter volatility classes, or between brands of gasoline from different refineries because of compositional differences. The differences are small and there is no practical way for the consumer to identify gasoline with a higher-than-average heating value.
FROM:Gasoline -- Is It All The Same? What about Octane?
For all intents and purposes, the heating value (BTU content) of the fuel doesn't change with octane rating.
(The change is so insignificant that it's not obviously traceable.)