Oil
The main criteria of a car’s eco-friendliness is generally seen as the energy it uses in its operation. Cars have primarily run on oil-based fuel in the form of petrol or diesel for over a century. But we are now at the point where demand for oil is rising so much, and reserves are declining, that oil is actually running out. Oil prices have always been volatile and they react to speculation about events ranging from terrorism to hurricanes, but rising demand and diminishing reserves mean that overall, oil is just going to keep on getting more expensive. However despite this, most cars today still rely on oil as their primary source of energy; and oil is obviously a key contributor to carbon emissions, and climate change.