The determination of the component fatigue life under cyclic loading with variable stress amplitudes is a problem of high interest for the automobile engineer. This paper is a theoretical study showing that the factors influencing the value of the expected life can be specified by the choice of a convenient mathematical formulation of the loading spectrum shape, of the S-N curve slope, and of an appropriate cumulative damage law. The final equation proved to be suitable for graphical presentation. That allowed construction of a very general nomograph for rapid informative calculations and thus allowed time for further quantitative analysis of the influence of every considered parameter separately, as well as for the formulation of requirements on the accuracy of their determination. According to these results, the influence of the spectrum width (maximum stress range) appeared as the most serious one followed by the influence of the unit spectrum size, of the slope of the S-N curve, and of the degree of its modification, while the influence of the spectrum shape (that is, of the changes inside the range of the exponential type of distribution and the normal one) was by far of a lower importance. This work can serve not only for the decision about the most effective way for redesigning a weak component but also for comparing two program load tests.