I have a theoretical question.
Suppose two cyclists of equal talent and fitness are riding together on paved roads. Rider A is riding a decent, $1500 road bike of 2005 vintage. Rider B is riding an old mountain bike, 1990 vintage, with knobby tires and platform pedals. Rider B is obviously going to be exerting significantly more effort to maintain the same speed.
The question is: what elements of the clunker bike, if changed individually, would have the most impact on increasing the speed of the clunker bike?
This isn't meant to be a practical question -- clearly, it would be better to scrap the old bike and buy new. It is more to examine what elements make a modern road bike faster, on the road, than a clunker bike.
My first-pass rating of changes most increasing of speed (ranked from greatest to least impact on speed), are as follows:
1. Change tires to narrower, high pressure slicks.
2. Change to clipless pedals and shoes.
3. Change to dropped bars, for reduced aerodynamic drag.
4. Change to 700 diameter wheels.
5. Change to lighter frameset.
Would be interested in hearing what everybody else has to say.