If you know the gear inches on your fixed gear, you should be able to determine what gear combo this is on your road bike.
If gearing is the same and you aren't as fast, then its either your position on the bike...crank length...or the bike difference themselves...like
tires or pressures. Its probably mostly position.
Most are faster on a road bike...but not by a lot. I have ridden with some fixed gear guys that routinely ride 21mph solo...strong guys.