I am convinced the reason Thailand uses this system is to collect fines from offenders of the arcane and hilarious rules, rather than customer convenience, as dealers claim, which has nothing to do with it.
When you buy a new car, you're sometimes given red plates. They are purportedly legal, and always temporary, but come with a slew of rules of which buyers are often unaware. You can't drive outside your home province without permission, you can't drive with anything in the bed of your truck ... the red plates are a red flag to the bulls at police checkpoints. Nobody along the way will inform you of these rules. Not the dealer, not your "new" insurance company, not the road tax people or registration branch. Further, nobody will tell you that you are required to carry, at all times, a copy of your car's blue-book with updated annual stamps to prove you're legal. They all leave this up to you to figure out -- the hard way. (Thai style)
But sometimes the dealership's sales are outpacing their supply of red plates. This is where they get creative. They give buyers a "letter" affixed with an official stamp to throw the money-grubbing cops off the trail. They promise buyers the next available red plates will be "theirs" and there are no problems involved with "driving home today."
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The inept system of red plates could easily be eliminated by placing the onus of immediate registration on the dealers. "No car or truck shall depart the leadership as "sold" unless it has legal, permanent plates." Cops can rest at ease. (never gonna happen)
I bought a truck last month. Was told the red plates would be replaced with fully-registered white plates within 30 days. Happy. Went home and made holiday reservations at the sea for the following month. At the thirty day point, called in to make sure white plates were imminent. NOPE, paperwork hasn't been sent in yet. Gotta pay extra for express service. Slippery Thai kunts -- liars.