Its technical design was an evolution of the successful Fiat 600. The internal name for the Fiat 600 development project was "Project 100" and consequently, the internal Fiat codename for the 850 project was 100G (G was a follow on of model designations for the 600 which ran from A to F). The engine of the 850 was based on that of the Fiat 600, but had its capacity increased to 843 cc. The 850 came in two versions: "normale" (standard) with 34 hp (25 kW) and engine code 100G.000 and "super" with 37 hp (28 kW) and engine code 100G.002. The maximum speed was approximately 125 km/h (78 mph). While it was not a large step forward in technical development, it possessed a certain charm with its large rolling eyes and its short tail, in which the engine sat.