What do you mean steam powered cars? When I think of an X powered car I think of putting X into the car to power it. You put petrol in a petrol car and electric power in an electric car. You don't put steam in a steam train, aren't they actually coal powered? According to Wikipedia, steam cars just ran on petrol, but the combustion engine was external, and the car was less efficient overall. Steam engines don't seem to make sense for cars.