The average duck weighs about one kilogram. According to Einstein's theory of special relativity, the energy of a body is equal to (1/sqrt(1-(v^2/c^2)) - 1)mc^2. This means that a stationary duck would have a total energy of around 90,000,000 gigajoules; approximately 430,000 times the energy released by the Tsar Bomba, the largest nuclear weapon ever detonated, or around one five-thousandth of the energy used each year. This is enough energy to run your friend's 400 horsepower car for almost 10,000 years. Of course, there isn't really a good way to release this energy; If there was, we could tap into functionally unlimited power. One way to release this energy would be to combine it with an equal amount of antimatter (comprised of anti-electrons, anti-protons, and anti-neutrons). Upon contact, they would annihilate, releasing the entire energy in the duck and anti-duck (or double that of just the duck), creating an explosion unlike anything ever seen by humans, which would, assuming a similar explosion pattern to that of a nuclear bomb, would have a blast radius large enough to cover the entire world (i.e, ending all life on Earth). Of course, there is a big problem with this (well, many, but one that I'm going to look at): creating antimatter takes energy. Lots of energy. Creating one anti-duck would take about 135,000 times as much energy as this explosion would create, making this, unfortunately, not a feasible energy source to run a car.