18-wheeler vs tractor-trailer

18-wheeler

noun
  • A big rig truck: a semi-trailer plus the truck or tractor pulling it. 

tractor-trailer

noun
  • A large truck consisting of an engine and cab section (the tractor) and a cargo-carrying semi-trailer or trailer. 

How often have the words 18-wheeler and tractor-trailer occurred in a corpus of books? (source: Google Ngram Viewer )