Power Generation and Vehicle Movement

Very little, unless the thing you are driving over slows the car considerably and extracts that energy.
It's a bit like asking how much force my coffee cup is generating by sitting on the desk.
There must be a reaction from the desk to support the cup, can't I remove the cup and harness this upward force....<scurries up to develop the negative cup propulsion system>
Del

If the it is a cat any living creature, the power/energy may be considerable - at least in dBs

And also with so many PETA activists around, you may really feel powerless (ie it has absorbed all your power).