Am I misreading the gp's cited chart? According to that, it seems like hydrogen is only a factor of around 4x worse in terms of volumetric energy density, and is better than petrol in weight density.
Weight density doesn't help you in a car; these aren't rockets where most of the weight of the vehicle is fuel. In a car, the weight of the fuel is barely noticeable. What's much more important is the size of the tank, and a hydrogen tank takes up a lot of room (to get decent range), and is also quite heavy to contain a pressurized gas that literally leaks out constantly because its molecules are so small.
