An electron swarm study using molecular gas-rare gas mixtures is briefly reviewed, and the advantage of using these mixtures to evaluate inelastic electron collision cross-section data for molecules through electron swarm study is explained. This advantage also suggests a new procedure for deriving a consistent set of electron collision cross sections for molecules by using electron swarm data measured in pure molecular gas and in the molecular gas-rare gas mixtures alternately. The procedure is detailed by using an example of C2H4. The derived cross-section set for C2H4 covers the energy range where a conventional electron beam experiment is not practical and can be crucial for the quantitative modeling of relevant plasmas.