The 250S was a one-time 6-cylinder experimental car bridging the gap from the 2-liter A6GCS/53 to the 3-liter 300S.

The 1954 Mille Miglia saw the participation of a special A6GCS with a 2.5-liter straight-6 engine derived from the 250F Grand Prix single-seater, de-tuned to 230 bhp to run on regular petrol, mounted to a 4-speed gearbox. The car raced again at the Gran Premio Supercortemaggiore (named after a make of petrol by its sponsor, AGIP) at Monza with Mantovani/Musso, who retired.

The project regained momentum in 1956 when revised technical regulations at Le Mans limited prototypes to 2.5 liters. Two engines were consigned to Automobiles Talbot of Paris and entered in the 24-hour race as Talbot-Maserati; both retired. At Modena a single A6GCS was rebuilt with de Dion rear suspension and a 4-speed transaxle gearbox in unit with the final drive for improved weight distribution.

After racing with little luck in places like Agadir and Dakar it was used by Stirling Moss as a race car for the Mille Miglia.