The 16th running of the Mille Miglia proved to be it's most popular to date with over three hundred official entrants. In order to maintain some type of control the cars were now painted with their starting times as their racing numbers. The autostrada sections were for the most part eliminated as a result of countless complaints from the drivers. The North-South leg of the race no longer took place over the classic route of the Apennines with the Futa Pass between Bologna and Florence, but along the Tyrrhenian coast. The race also reverted to the traditional distance of around 1600kms and the anti-clockwise direction.
The Ferrari lineup driving the brand new 166 MM consisted of works cars for Biondetti, Taruffi, Bonetto and Amateur driver Giovanni Vaccari. The MM stood for Mille Miglia and commemorated Ferrari's first victory in the Brescian marathon by Clemente Biondetti the previous year. Touring of Milan whose Superleggera designs were the lightest available, built a body over a 166S that had been shortened by 37 centimeters. The car was quickly dubbed the "Barchetta" or little boat. This roadster would later become a classic in Italian sports car design. According to some experts Ferrari had intended the 166 MM mainly as a customer racing car of which there were many takers, but when they discovered the potential a number of works cars were also constructed. The factory also had to deal with various customer cars that were serviced by the Ferrari factory. Alfa Romeo had an experimental 2500 cc Touring for Franco Rol. Further back in the field was British driver Donald Healey driving a car of his own make, the Healey - Elliot.
The first cars were again flagged off at one minute past midnight on Sunday morning. Of the three factory Ferraris it was Bonetto who led the way to Rome followed by Taruffi and Biondetti. The fourth car of Vaccari had missed the refueling spot at Tarquinia forcing them to stop at an ordinary gas station for fuel. The 12-cylinder engine was made to run on a specially mixed racing fuel and the "sick" Ferrari could only limp to Rieti where the tank had to be completely emptied before being re-fueled. Reaching Pescara, Bonetto was slowed by braking problems and was passed by the other two Ferraris. The leading Alfa driven by Franco Rol was now up to third place. At Ravena Taruffi held a six-minute lead over Biondetti only to drop out of the race with transmission trouble. For the third straight year Biondetti inherited the lead and for the third straight year the fifty-one year old driver held on to the finish for his unprecedented fourth Mille Miglia.
Bonetto was able to hold on the second place followed by Rol but the sensation of the race was a Fiat 1100 Stanguellini fourth an hour and fifty minutes after the leader and at an average speed of 71 mph. Biondetti's race winning Ferrari would later be purchased by Lord Selsdon who as a co-driver to Luigi Chinetti would win the 1950 24 Hours of Le Mans in that car.