Due to exponential increase in wind-resistance, it's not linear. Aero-drag increases to square-power of velocity. Going 55mph requires about 9bhp, less than 1-cylinder. Because going from 55 to 100mph would require:
[(100mph/55mph)^2]*9bhp = 29.8bhp (crank)
Or over 3x the power to go 1.8x faster. So it's actually down by -70% of peak-power.
