How do I cure my 100 pound Bull Mastiff puppy of trying to chase cars?

My 7 month old puppy started chsing cars this morning, we were out for our walk and 3 cars had already ped us. On the 4th, he made a break for it, and IS HE STRONG!! He surprised me and almost took me with him. I gave him a no and spun myself into the leash every time a car went by. Instead of walking our usual walk, I took him to local playground and had him chase and explore some, on and off the leash, and we practiced his commands and tricks, trying to use up some of his energy. The walk home was no better tho. He is on a choke style training collar, but I think I will buy a prong collar for walks, He is growing too fast for me, I am desperate...I need other ideas on how to break this habit