In usingEnglish.com I found,
Where the rubber meets the road is the most important point for something, the moment of truth. An athlete can train all day, but the race is where the rubber meets the road and they'll know how good they really are.
In the Phrase Finder I found this explanation too,
I remember that phrase from a jingle in a series of TV commercials in the 1960's and 70's. I think it was for Firestone tires, but I'm not positive. I remember that there was this tune, sung by a choir of deep-voiced men. I think it went "Firestone...where the rubber meets the road" Of course they may have used the phrase before then. "Where the rubber meets the road" refers to the tire of a vehicle on the surface of a road. It means "where it really counts."
What would be a good translation for the expression?
(I cannot think of any similar concept in Spanish, using "rubber", but maybe I am overlooking my "data-base")
I know "a la hora de la verdad" "el momento decisivo"...
Any other ideas?