What you say has merit, and to be honest I'm not that totally familiar with the Cars' output prior to HC other than their singles. Nevertheless, I always base my view of a band's popularity on this side of the water, and if you look at the below page from Wiki showing their discography, you can see that their first four albums, while big in the US certainly, made nary a scratch on the UK market. Even at that, their only big singles before HC were "Just what I needed" and "My best friend's girl" (1978) which reached 17 and 3 on this side respectively, and "Shake it up" (1981) which got to number 4 in the US. It's only when HC is released we see big hit singles in the US and much more airplay in the UK and Europe, leading to an album which went 4Xplatinum.
The Cars discography - Wikipedia, the free encyclopedia
I see what you mean about sales though; they were very steady on the US side weren't they? It's more a case of who knew the Cars (who wasn't a fan) prior to the release of HC, and for me as I say those two singles were it. After HC everyone knew "Drive" and the other singles to a lesser extent. Still, what you say is true, but certainly from at least a personal standpoint, this for me is when the Cars seemed to explode onto the scene. Remains one of my favourite albums without question.