best gears in rear with 4 spd?

here is the calculator i use. http://www.bigblockdart.com/forum/speedtire.php

if your running a 3.91, no OD and a 27" tire then at 75mph your turning about 3600 rpm. screw that.. i'm running a 28" tire and 2.94 gears and still feel i need an extra gear..:)