final drive

ok so this might be a dumb question, but i was looking at final drive numbers for the gt6 and the non overdrive mk2 had a 3.27 to 1 and the overdrive version had a 3.89 to 1 how did that happen? i assume they used a different rear end... but why? the reason i was looking at the number is i was trying to find some numbers on fuel economy so if you have any mpg numbers that would be nice too

thanks.

dont have one but i would wikipieda it