Main Idea:
I just wonder that the huge weight of trucks or cars should be somehow utilized to generate energy. My main idea is to make small air bags of some flexible material or kind of small pockets filled with air, on the periphery of the tyre. During the rotation of the wheel, when these bags will get in between the tyre and the ground, they will get compressed as the wheel continues to rotate. The compressed air from each bag should be collected through a duct and micro turbine should be placed in front of that duct to generate electricity. The battery of the vehicle could be charged with this electricity or perhaps the energy could be stored in some other storage device.. Another advantage would be that as the speed of the vehicle increases, the more quickly a particular air bag gets compressed and thus more energy could be generated.

Design Details:
One technical point is that once the air bag gets compressed, how is it going to be inflated again. This could be done by a proper design of the system. From one side of the wheel one main duct, let me call it as an Inlet duct, should connect each bag and this duct should be having a one-way valve. That is, the air could only enter inside the duct from the outside atmosphere. Similarly, there should be another duct, say Outlet duct, on the other side of the tyre which should only let the compressed air out to the turbine. When any particular air bag gets compressed, it will push the compressed air into the outlet duct. As a result the pressure within that bag will decrease and it will suck the air through the inlet duct.