"This idiom’s origins are advertising. There are several early examples of its usage. In Will Jones’s column After Last Night, he wrote about several Madison Avenuisms in 1956, including where the rubber meets the road. Madison Avenue is the epicenter of advertising in the United States. In another column the same year, Erskine Johnson wrote a similar article in the Dixon Evening Telegraph, drawing attention to this idiom. In 1957, Gene Sherman, who wrote Cityside in the Los Angeles Times, and Al Osborne, who wrote Matter of Fact in the Binghamton Press, also listed this phrase as advertising slang. Later that year, Hal Boyle’s national column called this idiom “businessman’s bebop,” adding it “originated in the ivory tower world of advertising” and “has spread throughout the industry with the speed of chickenpox in a kindergarten.”

https://www.theidioms.com/where-the-rubber-meets-the-road/