When you are using final gear on downhill might will help to exceed speed of 30mph.In realty 30mph is really fast speed for downhill you can not mainten the bike.This speed is really dangerous for handle the bike on downhill.You have to daily practice and some changes in you bike to do this thing.You need to match both the number of bolts and the "bolt circle diameter" which is most likely 110 mm on your crank