After looking into it, I too am a little surprised the 1954 Cadillac recommends an AC 46 heat range since from listening to other 50s Cadillac owners regarding AC, Champion, and Autolite as well as the specific recommendations of Autolite in their 1958 spark plug catalog, one heat range is specified for Cadillac 1949-54, then the next coolest range for Cadillac 1955, and finally the next coolest range for Cadillac 1956-58, so basically 3 ranges in three years. I am just wondering what factors are causing this drop in recommended plug temperature. Especially puzzeling to me is the drop between 1954 and 1955. Any thoughts would be appreciated. Jim