Now, I personally don't know, nor have I researched it that the number of the octane rating of gasoline is that number, say 87%, plus 13% of something else. I would say if a gas pump says 87 octane I personally would be in doubt that this means there is 87% octane by volume. As I said before, gas pumps might say octane=87, but it will also say something like (this isn't it, but I don't recall the exact formula: 87 octane computed by the RM/2 method. I have no idea what R or M represents, but when it says it is computed by a certain method, that means there is more than one method. Perhaps one method is used as a standard and by law must be used, but I don't know. But, consider this: Where does the energy come from by heating octane in the presence of oxygen (O2, molecular oxygen, not O, atomic oxygen). I won't attempt a guess at the exact chemistry, but there is potential energy in non-ignited octane. This energy is released by the breaking of the bonds in octane. It is the breaking of these bonds in a manner which is known as an exothermic reaction, meaning heat is released when the bond(s) break. There are other types of chemical reactions known as endothermic reactions that absorb heat, as a aside.