This thesis describes the development of a tank insert for diesel cars, from idea to a prototype. The idea is to design an insert which makes it impossible to refuel petrol in a diesel car. The insert shall be installed in the existing tank opening on diesel cars. The tank insert is designed for the Swedish company SverigeGrepen AB located in Vänge, just outside Uppsala. Their main product is a pitchfork that is ergonomically designed and durable.A pre-study were performed in which information relating to fuel caps, tank openings, petrol- and diesel nozzles were collected.