I would put a generator on the tire, so that when the engine is disengaged, the generator is engaged. Put an electric motor attached to a gear on the rear tire. Turn off the motor going downhill using a switch that opens when it is tipped in a certain direction (sorry, I don't know what to call it). The generator collects energy otherwise turned into heat and friction. Use that energy to assist yourself up hills and you will save quite a bit of gas.
At least thats what some hybrid cars do.