After an astronomical rise in its first day of trading, VinFast Auto Ltd. is now coming back to Earth — erasing a chunk of its billionaire founder’s fortune.

The Vietnamese electric vehicle-maker has fallen sharply over the two most recent trading days in New York, eliminating more than half of the 255% gain it notched on Tuesday when it listed on the Nasdaq Global Select Market.

At $46.4 billion, VinFast’s market capitalization is still just above that of General Motors Co.

Chairman and founder Pham Nhat Vuong, who controls all but a few shares of the company, has seen his net worth tumble roughly 40% to $26.2 billion, according to the Bloomberg Billionaires Index.

Big share price swings were to be expected. Vuong controls 99% of VinFast’s stock, mostly through his business conglomerate, Vingroup JSC. That leaves a small fraction left for other investors to trade, meaning that even relatively small transactions can have a big impact on the price.

Still, Vuong isn’t hurting. Because Bloomberg’s index hadn’t accounted for his stake in VinFast until the company this week completed its merger with a blank-check company, he remains significantly richer than before the listing.

On paper, he gained almost $40 billion in the first trading day — one of the biggest wealth jumps ever recorded by the index.