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The energy at the blade-air interface must be lower than that at the road-to-tire interface due to energy loss in the tire, gears, wheel bearings, lower sprocket, chain, upper sprocket, jack-shaft bearings, then the blades themselves.
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Sit down and enjoy this moment of enlightenment.
Power is force times velocity.
For the wheels it's simple: The velocity is just the difference in velocity of the wheels and the ground.
For the propeller it is just as simple: The velocity is just the difference in speed between the propeller and... (this is the facepalm moment!) the
air!
The propeller doesn't give a hoot about ground speed!
Let's say we wanted to go 10m/s down a 5m/s wind and were transferring 100N of force.
We harvest 100N*10m/s = 1kW at the wheels.
The useful power of our propeller needs to be: 100N*5m/s = 500W.
Ergo (I love that word) we need a total system efficiency of 50%, including all losses, that is very very much
not over unity, no magic needed!
Propeller efficiencies can be 82-92% according to the internet.