The plot thickens.....a quote from Hagerty Insurance regarding the authenticity of the Bullitt car :-
"One of the key shots of the chase is when the Mustang overshoots one of the corners. McQueen’s character, Frank Bullitt, throws it in reverse and seemingly smokes the left rear tire while backing up. That smoke, however, was fed to the wheel well by a trunk-mounted smoke generator via a duct patched into the fender liner."
It also states the the stunt car was used for the jumps and McQueens car used for the driving scenes.