Bearings Loss

20MPH 20secs

Rim, Tyre, Tube weight = 655g

20mph = 20 x 1609/3600 m/s = 8.94metres per second. 

Energy = 0.5 x 0.655 x 8.94 * 8.94 = 26 joules

Power loss = 26/20 = 1W

At 30MPH = 1.5W

For 4 wheels = 6W

This is 1% of total power