This is from memory and it may be a little fuzzy. In an episode of M3S I think it was Robbie that did a test drive of a 1965 or 1966 Pontiac GTO. As I remember the story Steve had a girlfriend that was in the advertising business and she brought this car home for the boys to tell them what they thought of it..... Am I on the right track here? I'd like to know the episode????

You were on the right track. Robbie is given use of the experimental sports car in the episode "From Maggie With Love" with Guest Star Dana Wynter, from the 1965-66 Season on CBS.