MGE said measures to fight the coronavirus outbreak and the closure of factories was the reason behind the steep drop.
In January-May, new car registrations were down 25% at 45,315.
Sales of light commercial vehicles dropped 45% to 1,279 in May, sales in the first five months were down 22% at 8,334.
Sales of heavy commercial vehicles were down 78% at 169 in May.
Motorcycle registrations fell 21% to 489 in May and decreased by 12% to 1,746 from January to May.
Earlier in April MGE said new car registrations could be at 118,000 in 2020, down from an earlier estimate of 155,000 for the year. Sales of light commercial vehicles could reach 22,000 and sales of heavy commercial vehicles 2,300.