Lightweight is an effective design strategy to conserve energy in automotive vehicles. It is a big challenge to evaluate the level of lightweight for passenger cars. This paper summarizes various evaluation methods for lightweight automotive vehicles. A lightweight index [Lv for internal combustion engine vehicles (ICEVs) and Lev for battery electric vehicles (BEVs)] is proposed to assess the lightweight of passenger cars. The proposed lightweight index is composed of the nominal density, weight-to-power ratio, and fuel consumption of footprint area (in the case of ICEVs) or electricity consumption of footprint area (in the case of BEVs). The validity and universality of the proposed lightweight index are demonstrated through a statistical analysis of 7018 ICEV and 326 BEV models. The calculation procedures of the standard partial regression coefficients of statistical multiple regression and elastic coefficients are employed in the proposed method. The results show that either Lv or Lev is most sensitive to the curb mass of the vehicles. The proposed lightweight index can help guide automakers in setting reasonable weight reduction targets during new product development. In addition, the proposed lightweight index can be applied to new hybrid electric vehicles with further efforts, to facilitate the development of lightweight automotive design.