avengence

A friend of mine got a 2001 mustang gt automatic and recently changed it to a manual. He had 4.10's in his car so he kept them in there when he switched to the manual. He put in a 3650 tremec which is stock for his car. Anyway, when he drove it, it was geared really high for some reason. Redline in first was 15mph.

So just yesterday I isntalled 4.10's in my 2005 mustang gt and its like 3.73's in his. Redline in first is probably 25mph or maybe more. What do you guys think about this? What would cause this? Maybe his transmission had its gears changed?