my "mild 440" (383 stroked to 438, eddy heads, XE275HL cam, 10.5:1 compression) was originally geared with 3.21s in the rear axle, 28" tires, and the TKO-600 with a .64OD.
at 65 mph, my rpm was down around 1700 and it was downright painful, just barely enough rpm to keep running. at 55 mph, I was better downshifting into 4th.
I swapped to 4.56 gears in the rear, and can still do 75mph at just over 2500 rpm (65 is about 2400rpm), and it does great. nice and smooth, and feels like I could cruise there forever.
