100% this is not really about investment advice. Since anything can happen. But a diverse, but small portfolio might do better than a simple savings account. But a $100 a month in a savings account is better than letting 24 - 36 months go by with nothing saved towards the car and having to cancel a deposit.I'm no investment expert but just want to offer another point of view. This plan is sound only if Tesla stock goes up between now and when you need to pay the car. The whole plan will backfire if Tesla stock goes down because not only will be you underwater, you will no funds to pay for the car because the funds are now locked up in Tesla shares which you cannot sell (without realizing a loss).
In essence if you saved for 72 months you have $7,200 dollars, plus sell you existing car and you have a great down payment for your Model 3.
I guess my post is really about creating a plan to buy a car, than complaining about specs for a car that's specs haven't been finalized.
Heck, if you save $7,200 bucks and don't like the car, then you get your deposit back and have $8,200!, then do the happy dance.