I don't really know how convoy works, but I assume its the same as rail. With rail they wait until they have 15 cars going to the same general area. (it could be V6's, GT's, Shelby, etc) then they load a rail car. That is why cars are said to be "lost" or held in area 51, for long periods of time. you could have 14 cars destined to LA, and they will sit until 1 more car is produced for that area. I assume with convoy it is the same but I believe the number is usually 8.