What is 91 octane?

Octane, technically speaking, is the number of octane atoms in a molecule of gasoline. Gasoline ( we are leaving out the ethanol component's effect ) is a mixture of several hydrocarbons including heptane, benzine, and others. The number of octane atoms ( or its equivalent in the case of ethanol blends ) in the gasoline give it the number you see at the pump. It is a measure of gasoline's ease or resistance to burning when air and heat are present. High performance engines use high octane gasoline because they can compress the fuel more before igniting the mixture creating more power; if lower octane fuel is used, the knock sensors detect this and retard the timing to avoid engine damage which is why your mileage drops. Conversely, fuel mileage does not increase by burning a higher octane than needed; the higher octane's ability to resist ignition works against you and incomplete fuel combustion can result. The unburned fuel is dumped into the catalytic converters where it is burned, fouls the catalyst, and can cause failure of the converters.