Originally Posted by
kingfishr
The only relevant component is air resistance on the flats, not the weight. So if the guy lost 50 pounds but had the same aero-profile his speed would be unchanged. And if he gained 50 lbs and had a better aerodynamic profile his speed would increase.
If our hypothetical roadie is putting out 160 watts and weighs 150 pounds he'd be going 19.4 mph on the flat road.
Give him another 50 pounds and he's going 18.6 miles per hour.
estimations from an online power calculator (21 pound road bike, on the drops, everything else default values)
.8 miles per hour difference is not nothing!