18-wheeler vs freighter

18-wheeler

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

freighter

noun
  • One who loads a ship, or one who charters and loads a ship. 

  • One employed in receiving and forwarding freight. 

  • An aircraft used mainly to carry freight; a cargo aircraft or cargo plane. 

  • A vessel used mainly to carry freight; a cargo ship. 

  • One for whom freight is transported. 

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