Tesla Inc (NASDAQ: TSLA ) has not been valued by conventional measures in years, so the 20% “collapse” in the TSLA stock price since mid-September, which accelerated after an earnings disappointment Oct. 31, may not represent the end of the ride.

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Tesla was due to open for trade Nov. 13 at about $302 per share, having traded at over $380 per share as recently as Sep. 18.

Even at its current price, however, the company is worth $51 billion, more than four times its anticipated 2017 revenue of under $12 billion.

Contrast that with General Motors Company (NYSE: GM ), which is now worth $60 billion but on annual sales of $166 billion. Or Ford Motor Company (NYSE: F ), worth $48 billion on annual sales of $152 billion.

In other words, if TSLA stock is going to be valued like a regular car company, it still has a long, long way to fall.

Dog Stocks Love Trucks

CEO Elon Musk keeps investor excitement high with product announcements.

His latest, expected Nov. 16, is a semi-truck, competing with Navistar International Corp (NYSE: NAV ), which makes the International line, and Daimler AG (ADR) (OTCMKTS: DDAIY ), which owns the Freightliner brand.

Big trucks represent a much smaller niche than passenger cars, where Tesla continues to report production problems. After its latest loss, it bought Perbix, a machining supplier, to own “the machine that makes the machine” for auto building.

Musk now admits that scaling production of cars like the Model 3 is proving difficult, something I have been writing about for months. As exciting as self-driving and electric cars may be, making millions of cars is a major technological challenge companies like GM and Ford have mastered but Tesla hasn’t. If it can’t master this challenge, it can’t possibly justify its $51 billion market cap.

Is the Jig Up for TSLA Stock?

Tesla has accomplished an enormous amount, forcing change on Detroit, which long resisted it. But that change is now coming, and Tesla does not seem ready to respond.

Utility industry policies, implemented by states around the country, are quickly making Tesla’s planned niche in residential solar disappear. The battery business is going great, but as its latest earnings letter notes, this is just 15% of total revenues.

Traders don’t know what to expect from the fourth quarter, due to be reported Jan. 31, except for a big loss of $3.75 per share. Tesla has long used the currency of its strong stock price to leverage its borrowings. In September, it had $10 billion in debt on $28 billion in assets, up from $2.7 billion in debt on $12.5 billion in assets a year earlier. That debt to assets ratio remains much lower than its competitors, but it’s heading in their direction.

TSLA stock is speculative, the speculation being that Elon Musk can transform the car industry and dominate it. That industry has been transformed, but Musk is not dominating it. I have warned that TSLA stock is a bubble, and that the bubble will pop at some point as the valuations of auto companies approach one another.

The Bottom Line on TSLA Stock

Musk keeps Tesla running by continually pulling rabbits out of his hat that keep investor excitement high. The semi-truck is one example.

A self-driving all-electric truck sounds exciting, but what kind of range can it have? And how much time would it have to be out of action while being recharged? How much will it cost? These are practical questions.

Elon Musk has proven great at the vision thing, but Tesla has yet to be proven great on the practical questions.

Dana Blankenhorn is a financial and technology journalist. He is the author of the historical mystery romance, The Reluctant Detective Travels in Time, available now at the Amazon Kindle store. Write him at danablankenhorn@gmail.com or follow him on Twitter at @danablankenhorn. As of this writing, he owned shares in F.