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I reviewed my Tesla MVPA this evening after they reassessed my trade. I noticed that the GET is based on the full value of the Model Y.
I could've sworn that this wasn't the case the last time I traded in when I picked up my 4Runner in 2015. Sure enough, I looked back at the purchase agreement and the General Excise Tax (GET) that I paid was calculated based on the difference between the selling price of the new car less my trade.