A ST plate is actually for semi trailers. It combines tractor and trailer weight to the tractor plate. So if you put a ST plate on you boat trailer. It combines your truck and boat trailer weight to the truck plate. So if your truck and boat trailer weigh under 8k then your good. But heres the kicker. If you get stopped by the right cop or game warden they can call DOT in and inspect your truck and trailer. Will your trailer be legal in other states with a ST plate? No idea. In Illinois it is if want to take the risk. I called the local DMV down here and they all told me the same thing. Yes its legal. Thats what i gathered from asking round.
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