auto racing vs horse race

auto racing

noun
  • Any of various sports in which automobiles (motor cars) are raced, either around a track, on roads or across country 

horse race

noun
  • A competitive race for horses carrying jockeys; often the subject of betting 

  • An exciting and arduous competition (as in a political campaign). 

How often have the words auto racing and horse race occurred in a corpus of books? (source: Google Ngram Viewer )