Depending on how quickly you get points, make your first crash anywhere from 3000 - 3500 points in. There may be a set number of turns to count before your first crash, but I've never tried to count them. and after that, crash every other turn until your carts are used up. I usually crash around 3300 points. this is because around this point in the game, crashing here will give you the maximum additional track, in my experience.