How big are the car files? I haven't played with it recently, but I had to reduce the size of my cars to 1024x1024 and then reimport them into the game to keep it from lagging, It's just an intense mod. I would reduce car sizes and crew sizes, either by exporting them out of the game and then resizing them in photoshop or whatever and then importing them back in. Or you can try winmip to do that too. I haven't used winmip in a long time, so I would not be the best resource for that.
I think I also saw someone say to turn off the car shadows too.
Also, what track are you trying to run it on? That could be your issue too.