1. A method for detecting leakage from an evaporative emission space of an automotive vehicle fuel system that includes a tank for holding volatile liquid fuel, the method comprising: sensing each of plural physical parameters characterizing a gas mixture that includes volatized fuel, in the evaporative emission space; measuring a beginning value of a first of the sensed parameters at a beginning value of a second of the sensed parameters at a beginning test time; measuring an ending value of the first parameter at an ending value of the second parameter at an ending test time; deriving an actual measurement by processing the beginning and the ending values of the first parameter and the beginning and ending test times using a relationship that correlates difference between the beginning and ending values of the first parameter, difference between the beginning and the ending test times, and difference between the beginning and the ending values of the second parameter; deriving a theoretical measurement that represents the value of an actual measurement resulting from the step of deriving the actual measurement in the absence of leakage from the evaporative emission space; and comparing the derived actual measurement to the derived theoretical measurement.