Here's What's Pushing Tesla Stock Toward $600 -- and Beyond - Motley Fool

Musk himself has been among the biggest winners in Tesla stock, vaulting to the No.

Specifically, the biggest driver of Tesla stock between now and the end of the year is likely to be the decision from S&P Dow Jones Indices to add Tesla to the S&P 500 index.

S&P Dow Jones Indices reports that $11.2 trillion in assets uses the S&P 500 as a benchmark for return comparison purposes.

Take that amount and apply it to the $4.6 trillion in S&P-tracking assets, and you get $46 billion to $69 billion in Tesla stock that index funds will have to buy come late December.

That amount is so huge that S&P Dow Jones has looked at spreading out Tesla's inclusion over a couple of days.

As of the end of October, nearly 48 million shares of Tesla stock -- worth more than $25 billion at current prices -- were sold short.

Admittedly, after the big gains in Tesla shares this year, anyone selling the stock short has to be prepared for huge risk, and so they're less likely to get squeezed out than most short-sellers would.

However, if you think Tesla's underlying business justifies the big run-up in its stock price this year, then you might prefer not to wait until the index funds buy in.