Shares of electric vehicle manufacturers NIO (NIO 2.30%), Lucid Group (LCID 0.83%), and Rivian Automotive (RIVN 1.03%) are down sharply from their 2021 highs -- 58%, 75%, and 81%, respectively. That makes them very attractive when you consider the near certainty that EV sales are going to continue to grow. But when comparing those companies to legacy automakers Ford (F 0.17%) and General Motors (GM -0.05%) on this one key metric, there's something investors should be very cautious about. In this video, Fool contributors Jason Hall and Jeff Santoro talk about that very important number.

*Stock prices used were the afternoon prices of Sept. 21, 2022. The video was published on Sept. 22, 2022.