Have you ever wondered by how much CO2 emissions can be reduced by running cars electrically or with hydrogen as fuel? This Brief provides a quantitative answer to this question using the example of the combined road traffic in Spain. The authors calculate the resulting greenhouse gas (GHG) emissions for the production of hydrogen gas or the required electricity and installing and maintaining the necessary infrastructure. In this way, they can compare with the GHG emissions in the present situation of oil fueled cars. Using different scenarios, they obtain an assessment how much `greener' the electric or hydrogen cars can get.The method described in this Brief is scalable and readily adaptable to other countries. It can thus be used for investigating sensible approaches and developing recommendations for a conversion. As expected, the results depend strongly on the production scenarios for hydrogen or electricity production. The wrong choice can even result in increased GHG emissions. A proper choice of the roadmap toward a more sustainable and greener future is of greatest importance - the results described in this Brief can serve as a valuable and useful guide on our way.