Dumb criminals do exist, but I think someone would be extremely fortunate to have the carjacker initiate the car jacking from a position where he can be run over. Flooring it does create a moving target but it probably doesn't do a whole lot to disrupt the assailant. In other words he can start shooting you.
IMO, I think it's better tactics to engage the assailant. As has been discussed on this forum before, it's a lot harder to shoot someone while their shooting at you. Every scenario is different but I think in many car-jacking scenarios the best tactic is to engage the assailant. If it doesn't put him down, hopefully it has him jumping around and /or running and that might be the time to flee.