David Letterman loves his Tesla Roadster so much that he invited Tesla CEO Elon Musk onto the Late Show last night. The question he should have asked: How long will Musk keep his job?

Mostly Letterman wanted to know why Detroit's big car companies didn't come up with mass-market electric cars, and whether Tesla's Roadster really would save the planet. (He made a good point about carbon emissions from coal-fired electrical plants.)

But Letterman, when he let Musk get a word in edgewise, let him off easy. He didn't quiz Musk, for example, on whether the Model S show car Musk drove on set was the real thing. According to Dan Neil at the Los Angeles Times, it's not. The slapped-together prototype, a rebuilt Mercedes with a Tesla-designed powertrain, is "just barely ambulatory — more like a glorified golf cart than a harbinger of tomorrow tech," Neil wrote. And Tesla executives confessed to Neil that the car was far from being finished in its design, let alone production.

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Here's another thing Letterman should have asked about: How is Musk going to build the Model S? Even if Tesla gets the $350 million in government loans it's hoping for — far from a sure thing — it will fall hundreds of millions of dollars short of the real cost of bringing the Model S to market. An insider tells us Tesla is about to close a new round of financing from a so-called "strategic" investor — that is, some industry powerhouse, rather than a traditional financier. Tesla almost ran out of money last fall, and has run on fumes since then, despite raising a $40 million round of convertible debt from existing investors.

Any new money will mean handing a large stake to the new investor. Daimler, which already has a deal to buy parts from Tesla for its own electric car, is a strong possibility. But will they leave a hothead like Musk, with his habit of stretching the truth, in charge? That's what Letterman should have asked — not if electric cars will come to market, but if Musk will be the man to do it.

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