The Model 3 was followed by another unexpected winner, the Ford Kuga compact SUV, as crossovers and SUVs took their highest market share in Europe, at 42 percent.
Matthias Schmidt, an independent automotive analyst in Germany, attributed much of the Model 3's surge to demand from the Netherlands, where company car taxes for electric vehicles increased on Jan. 1 to 8 percent on cars under 45,000 euros from 4 percent on cars under 50,000 euros.
Tesla sold 12,053 Model 3s in December in the Netherlands, according to the European Alternative Fuels Observatory, an EU-funded statistics service.
In addition to the Dutch company car tax increase, the shakeup in the sales leaderboard was largely due to other "artificial" conditions, according to JATO.
Europe's new-car market showed big growth in December, with registrations up 21 percent year-on-year to 1.26 million, according to JATO.
Facing a new CO2 emissions fleet average of 95 grams per km for 2020, many automakers sought to register as many higher-polluting models as possible before Jan. 1. That sales push included self-registrations, JATO said.
"The growth is most likely due to self-registrations from those hoping to resell the units as second hand in 2020," said JATO analyst Felipe Munoz. He cited Spain as an example, where total registrations increased by 6 percent, but business registrations increased by 23 percent, and private ones fell by 7 percent.
"The December registrations are an indication of the ongoing regulatory challenges facing carmakers in the year ahead," Munoz said, noting that overall CO2 emissions rose in the fourth quarter of 2019.
December's sharp increase was not unexpected. Peugeot CEO Jean-Philippe Imparato told Automotive News Europe last fall that he thought December would be a record-breaking month.
The gain was also magnified by a decline in the fourth quarter of 2018, industry group ACEA said last week, as some automakers pulled uncertified models off the market after the WLTP deadline.
Kuga, Duster and Qashqai surge
With the unusual market conditions as a backdrop, some models recorded large gains.
Ford Kuga sales were up 127 percent to 22,045 for the month, with a redesign that Ford described as "friendlier" new to the market.
In fifth place, the Dacia Duster small SUV was up 59 percent to 20,933 sales.
Other big gainers among the top 25 best-sellers were the Nissan Qashqai compact SUV, up 47 percent to 19,726 in seventh place; the BMW 3 series, up 127 percent at No. 20; the Suzuki Vitara small SUV, up 133 percent at No. 22; and the Smart FourTwo minicar, up 174 percent at No. 23.
Electrified vehicles recorded big gains across the hybrid, plug-in hybrid and full electric categories. The new Toyota Corolla led among hybrids, whose sales increased by 41 percent for the month, and plug-in hybrid sales were up by 80 percent, led by the BMW 3 series. The Tesla Model 3 represented 43 percent of battery-electric vehicles.
"Tesla continues to have an enormous impact on registrations at the end of each quarter and the Model 3 has put sedans back in the spotlight," Munoz said. "The question is whether this trend is sustainable, or will electric SUVs hinder the popularity of Tesla Model 3 in the coming months?"