Bumeington said:Just assumed W/(kg of body weight + bike) should be the same*, so 5.96W/kg for 68kg is 405W which is 5.41W/kg for 68+6.8=74.8kg. 5.41W/kg for 51+6.8=57.8kg is 313W which is 6.13W/kg for 51kg.
This has to be true, P = Fv = mgv --> P/m = gv (P - power, m - mass, v - speed, g - gravitational acceleration). So for the same speed v uphill and the same gravitational constant g, P/m should be the same, and the mass lifted is rider + bike.
There's more than just gravity (rolling resistance and air resistance), so the difference is bigger.