18-wheeler vs tractor

18-wheeler

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

tractor

noun
  • A truck (or lorry) for pulling a semi-trailer or trailer. 

  • A vehicle used in farms e.g. for pulling farm equipment and preparing the fields. 

  • An aeroplane where the propeller is located in front of the fuselage. 

  • A British Rail Class 37 locomotive. 

  • A movable coop without a floor to allow for free ranging. 

  • Any piece of machinery that pulls something. 

verb
  • To move with a tractor beam. 

  • To prepare (land) with a tractor. 

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