Air-resistance is a reality that we have to examine, our top-speed on a bike is dictated by it on the flats and on the downhills. It's a reality in motorcycles and autos as well. While power-to-weight ratio dictates acceleration, power-to-Cx*Cd determines top-speed.
It's not so much weight, but pounds/sq.in. of force pushing on the frontal area that overcomes the drag. The more pounds of force you have for each sq.in. you block, the faster you go, or for the same amount of force, the less sq.in. you have blocking the air, the faster you go.
For example, when I jump out of a perfectly good airplane and arch out flat with my arms & legs extended, I represent a certain cross-sectional surface-area. I have only my weight to push against the air that's flowing in that cross-sectional area, and it balances out at 125mph. Imagine that I'm carving a virtual-tunnel through the air by pushing it aside from the path that my body takes.
But if I straighten up and go head-first straight down, that same weight (force) against a much smaller cross-sectional area will allow me to push aside less air faster. I create a smaller tunnel through the air, have to push less of it aside and I end up falling at 200mph. Skin-suits and cone-head helmets can increase that to 220mph.
Same thing on a bike, you guys can test it out by cresting a hill at 10mph and sitting up and holding your arms out wide and you'll see a certain terminal velocity. Tuck down low the next time starting at that same 10mph and you'll get much more speed. The fastest I can hit on San Marcos Pass around here is 50mph on 10% grade in a tuck with my hands on either side of the stem with my face and teeth dangerously close. I can actually hit 52mph with a one-arm tuck, placing one arm behind my back with my hand directly behind my butt, and it's a little scary for control. Tucking in behind a car or truck on the downhill, I can get up as fast as them easily, 60-65mph. But most drivers freak out when I do that and they slow down, bastards!!! I have no idea how someone can reach 68mph. That requires 2.5x the power as 50mph and maybe if you're a 425lb guy, you might have enough mass behind you to push aside the air that fast... but I'd be damn hell scared of the brake-pads melting!!!
For the guy who can't hit 40mph on the downhilll, sprint as fast as you can as you crest the hill and start down. The faster you can get going at the top, the faster you'll be at the bottom.... and let go of the brakes!