What does the Reid vapor pressure test measure?

The Reid vapor pressure test is designed to measure the vapor pressure of gasoline, which indicates how readily the fuel will vaporize at a given temperature. This is particularly important in determining the tendency of gasoline to vapor lock, a condition where the fuel vaporizes in the fuel system, causing an interruption in fuel delivery to the engine. A higher vapor pressure means that the fuel is more prone to vaporizing under normal operating conditions, potentially leading to vapor lock, especially in hot weather. Therefore, the Reid vapor pressure test provides critical data for understanding the behavior of gasoline in varying temperature conditions and its reliability in engine operation.

The other options pertain to different fuel properties not measured by the Reid vapor pressure test. Density relates to the mass per unit volume of the fuel, while the octane rating assesses the fuel's resistance to knocking. Flammability is a broader characteristic that involves various factors, including vapor pressure, but the Reid vapor pressure specifically focuses on vaporization tendencies.