Lonnie Utah said:
The answer to your questions lie within....
http://www.kreuzotter.de/english/espeed.htm
I ran the numbers for you. With no wind on flat ground, at 180 it will take you 247 watts to go 20 mph. By losing 20 pounds will gain you 0.4 mph with the same 247 watts. *** the earth upwards and you'll see bigger gains....
L
The calculator I use agrees with your 247 watts at 20 mph for a 180 lb rider sitting upright, but only says 20.13 for a 160 lb rider (a savings of .13 mph on the flat). As you know, weight just doesn't matter a whole lot on perfectly level ground.
Up steep hills though (less than 10 mph), speed is almost proportional to weight. EG, on a 10% grade at 247 watts, dropping from 200 lbs total (bike + rider) to 180 lbs, the calculator says you go up from 5.31 mph to 5.87 mph. That's a full 10% faster, about the same as the ratio of 200/180 lbs.