Short answer: "130+" according to a study done by the College of the Desert and Sunline Transit Agency

Longer answer: The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes "knocking" in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting compared to higher octane grades (like "super" 93-octane gasoline).

The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that for gasoline, it costs more.

Hydrogen has an octane rating of 130 because it can be compressed more than gasoline and 100% octane before the fuel automatically ignites in the engine. (Gasoline with 87-octane has 87% octane, a special kind of hydrocarbon that makes up gasoline and other fuels).

Here are some other octane ratings:

• Methane: 125
• Propane: 105
• Octane: 100
• Gasoline: 87
• Diesel: 30