A water-fuelled car is hypothetical car that uses water as fuel by producing hydrogen and oxygen on board through electrolysis without external energy input. The document discusses several claims of individuals having created functioning water-fuelled cars using technologies like electrolysis, hydrogen boosters, and water to gas systems. However, the scientific community has not approved an actual water-fuelled car due to failures in violating thermodynamic laws or fully demonstrating the technologies. While more development is still needed, hydrogen boosters and water to gas systems may have potential future for enhancing fuel efficiency and reducing emissions without requiring engine modifications.