They’ve quietly blown the Big Three’s doors off. It rarely turns a profit, but electric car pioneer Tesla’s pegged by Wall Street as the most valuable U.S. automaker. When trading closed Monday, Tesla’s shares reached a staggering $312.39, taking their market cap to $51.5 billion and sending them past General Motors for the first time. A week ago, Tesla surpassed Ford to become the second most valuable car company. While some analysts predict share prices could hit $500, others say the rapid climb is symptomatic of a bubble — inevitably leading to a crash.