The phrase “the rubber hits the road” means when something begins, gets serious, or when an idea is put into practice.  Its source, I think, is obvious – it must refer to where an automobile tire contacts pavement and, therefore, has traction.  But I wondered how well it was documented.

Know Your Phrase reports the earliest written usage was in a 1956 newspaper article, where the phrase was stated as “when the rubber meets the road” and notes The Modern American Usage: A Guide, first published in 1966, mentions “the rubber hits the road” was gaining popularity.

Wiktionary lists a book from 1928, How to Avoid Automobile Accident: “Even 500 feet probably wouldn’t allow you to brake to a stop, because it’s ‘where the rubber meets the road‘ that counts.”

American Culture Explained (now there’s an ambitious website) states that racing is the source, though they offer no reference.

One thread of comments mentioned a recent version: “when the rubber hits the sky“, used for a humorous, purposely-mixed metaphor meaning “where practicality meets pie-in-the-sky visions.”

Since the first mass-produced car – the Ford Model T – was introduced in 1908, it seems that the phrase didn’t take long to become popular.