Had mine installed a week ago. They don't fold completely flat, but I'm not sure that I can think of a realistic circumstance where it's any less useful than with the old seats. There's still a ton of room there, and it's not like people are going to be using a Tesla for hauling sheets of plywood.
Actually, I do use my Tesla like a truck to replace my SUV. I have the rear seat down a lot and carry my gear and all sorts of big things.
More room in my car than my big Navigator actually!
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I would still like to see Tesla offer older cars the option of swapping out the really uncomfortable 2013 seats with these newer NG seats.
Obviously it is possible. Why don't they do it I ask?