If I have to answer, I’d say $1.25MM vs $9k is more dissimilar than Tesla vs GM. But unless you believe that both Tesla and GM won’t grow sales, comparing market cap to cars sold in the last year(s) has only limited usefulness.
If we assume Tesla will grow units sold while GM will remain about static AND we assume GM is fairly valued (while assuming unit margins are the same for both companies), then it tells us Tesla needs to sell about 140 times as many cars per year in the future to justify its current stock price. Now, if you assume Tesla’s cars will have a margin 4x as large as GM’s and that GM’s business is undervalued by half, then if my math is correct, Tesla needs to sell only 17 times as many cars per year ($1.25MM/4=$312k, $9k*2=$18k, $312k/$18k=17). That 17x increase doesn’t seem totally unreasonable.
Of course, this also assumes that Tesla will make its money off of cars sold and not some other wild Muskian idea: if he develops world-beating self-driving software selling at 90% gross margins or wisely acquires other companies like Henry Singleton did at Teledyne, we could all be saying a $5 trillion market cap is cheap for Tesla in a few years! That said, as an investor, I wouldn’t buy today’s Tesla at even half its current stock price nor would I bet against Elon Musk (especially by shorting!). A lot of things have to go right for Tesla to justify its stock price; I just don’t have any way of predicting what Musk will do next!