While I understand that 4X4s with bigger engines are less efficient than many smaller cars, surely if a person drives a large car but only uses it infrequently for shortish trips then they are polluting the atmosphere less, and using less fossil fuel, than someone with a smaller car who uses it for long distances all the time?
So, in principle, it is the only the amount of petrol used per year by a driver (regardless of the size of the vehicle) that has a greater or lesser effect on the environment is not?