"It's driven by the overall economy," he said, and called 2017 a "fantastic year" for Porsche, a unit of Volkswagen.
Meanwhile, competitors are struggling. Fiat Chrysler's sales fell 13 percent in January and another 1.4 percent in February. Ford's sales dropped 6.6 percent in January and 6.9 percent last month. And General Motors' sales were up 1 percent in January, but then fell 6.9 percent in February.
U.S. auto sales likely rose 0.4 percent in March, compared with the same time last year, J.D. Power and LMC Automotive said Wednesday. But the increase is most likely caused by increased consumer discounts, something analysts say is not sustainable. Sales are expected to drop even further for the remainder of 2018.
Zellmer said the majority of the subscription program's customers choose the more expensive tier, giving them access to 22 different models of high-performance sports cars and SUVs that can be "flipped whenever you want." And more than 50 percent of subscribers, he said, are new to Porsche.