well dosent the size of the pulley change the speed and the amount force needed to move it, take a bike and break down the different tires, thin light wieght tires turn faster more easier than chucky, a large road tire on bikes takes more to start it going but also makes it easier to spin at high speeds but a small wheel the power spinning it has to work harder to make it spin at high speed but has better low speed response (also take into account the different gears on the bike, aerodynamices, size on rider, etc...) i think there is a lot of variables that come into acount on this so untill i see a dyno chart that proves any major gain i think its a waste of cash and gas is expensive now... tear (armstrong pyhsics here)