Hey i go about 104-110rpm in a gear of 20 inches and go about 15mph is this good?

and one more thing i was thinking i own a mtb and a road bike and in a gear of 80inches on my road bike i can fly but but to get my mtb to go 15-20mph or to get it to move i got to go to a 20.3 inches gear and i was thinking why is this so