It was an exercise in stupidity.

An iron-pumping NYPD deputy chief — nailed last year for illegal use of steroids — has now been caught driving his department car to the gym for workouts, sources told The Post yesterday.

Controversial cop Michael Marino allegedly drove his unmarked car to the New York Sports Club in Bensonhurst, Brooklyn, at least six times. It is against department rules to use an official car for personal business.

Marino, 53, second in commander of Patrol Borough Brooklyn North, was transferred to Staten Island late last year.