100 pound loss = how much HP gain???

I had to think back where I heard the whole 10 pounds equals a tenth thing, but found an old Hod Rod magazine article from 2003 that mentions it. I'm sure there are older articles, but still most likely one of many sources. Kind of a fun read anyway, since they use a Duster as a example.

https://www.hotrod.com/articles/weight-reduction/