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You may have heard the term specific gravity in relation to gasoline. Most race organizations specify the type of gasoline by this method. Specific gravity is the comparison of the weight of fuel in relation to the weight of water. If water is rated as a value of one, then gasolines (since they are lighter) are specified as a percentage of 1. Many street unleaded 92-octane gasolines measure from 0.75 or higher, while leaded race and unleaded 87-octane street gasolines can test below 0.75. This is important, since the specific gravity of a particular fuel affects the mass of fuel that can pass through a jet orifice. Specific gravity of fuel also determines how high (or low) a carburetor float sits in the fuel. Therefore, switching from a high specific-gravity fuel to a low specific-gravity fuel will have the same impact as reducing the jet size in the carburetor, since fewer hydrocarbons will be introduced into the engine even though the jets have not been changed.