Originally Posted by
prathmann
No, if the top gear is the same (expressed in either gear-inches or in development) then the speed at a given rpm will also be the same. E.g. my Bike Friday with 451mm wheels (20" outer tire diameter) has a top gear of 109 gear-inches using a 60t ring and 11t cog while my Cannondale road bike with 700c (622mm) wheels has almost the same top gear (110 gear-inches) using a 53t ring and 13t cog. I could certainly get even higher gearing for both bikes, but the current high gear is already only useful for downhill runs or when in a fast paceline since I'm not strong enough to spin that gear on flat ground. Ultimately the highest useful gear on a bike is determined by the legs of the rider, not the size of the bicycle wheels.
When I said "same gearing," I meant as in a 53t big ring and 11t cog; that gearing (gear combo?) at the same RPM, will give a slower speed on the 20er compared to a 700c. I was trying to figure out what the OP was getting at; the "how much slower" question didn't make sense to me unless we arbitrarily and artificially restrict gearing to single speed at a fixed RPM.