What are the differences in the gasoline grades?
(Q&A by Ryan Learn)

The grade of the gas is indicated by an octane rating. The octane number is a standard laboratory measure of the fuel's ability to resist engine knocking during combustion.

A knock is a harassing trait in engines that is linked to gasoline and is characterized by a ping in the motor during operation. This results in a loss of power, poorer mileage, and a lack of smooth engine performance.

A single-cylinder four-stroke engine of standardized design is used to determine the knock resistance of a given fuel by comparing it with that of primary reference fuels. The primary reference fuels are composed of varying proportions of two pure hydrocarbons, one very high in knock resistance and the other very low. The highly knock-resistant isooctane (2,2,4 - trimethylpentane, C8 H18) is given a rating of 100 on the scale and normal heptane (C7 H16), with a very poor knock resistance, represents a 0 on the scale. Octane number is defined as the percentage of isooctane required in a blend with normal heptane to match the knocking behavior of the gasoline being tested.

For fuels with a rating higher than 100 octane, the rating is obtained by determining the amount of tetraethyllead compound that needs to be added to pure isooctane to match the knock resistance of the test fuel

.http://www.earlham.edu/~chem/pages/gasoline/grades.html