 Complete the following phrase:  In the early years, now legendary races such as the Mille Miglia, Carrera Panamericana and Targa Florio were part of the calendar, alongside the 24 Hours of Le Mans, the 12 Hours of Sebring, the Tourist Trophy and Nurburgring 1000 km. Manufacturers such as Ferrari, Maserati, Mercedes-Benz, Porsche, and Aston Martin fielded entries, often featuring professional racing drivers with experience in Formula One, but the majority of the fields were made up of gentleman drivers privateers in the likes of Nardis and Bandinis. Cars were split into Sports Car and GT production car categories and were further divided into engine displacement classes. The Ferrari and Maserati works teams were fierce competitors throughout much of the decade, but although Maserati cars won many races the make never managed to clinch the World title. The Mercedes-Benz work team pulled out of the championship after 1955 due to their 