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.

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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