[olddog]

I have a .64 5th in a TKO 600 behind a 5.0 stroked to 347. It had a 3.50 rear and 26.3" tires. Cam is 236/242 duration at 0.050" lift and a 110 lobe separation angle.

4th gear 2550 rpm = 57 mph -- this is a bit too load to talk.
.64 5th is 1632 rpm @ 57 mph -- cam does not like this at all.
.82 5th is 2091 rpm @ 57 mph -- this is perfect in my mind.
.82 5th is 2400 rpm @ 65 mph -- too much wind to talk anyway.
.82 5th is 2550 rpm @ 70 mph -- too much wind to talk anyway, but this is same as 4th 57 mph.

If you have enough low end torque, a little taller rear gear would work well with a .82 5th gear.

I had to rebuild my rear end (pinion bearing), so I swapped out to a 3.92 ratio.
.64 5th is 1835 rpm @ 57 mph -- cam is happy.
The problem:
4th is 2850 rpm @ 57 mph -- Way too loud to talk, now useless to cruise with.
2100 rpm in 5th is 65 mph
2250 rpm in 5th is 70 mph

So bottom line, the 3.93 rear gear makes 5th OK, but I still hate with a passion the huge step down in rpm when shifting into 5th. I cannot say enough times how much I hate this. Not to mention that 1-4 are too low now. I like the 3.5 ratio much better, in the lower gears.

I will eventually change to the .82 5th gear and have the synchronizers upgraded. I will most likely go back to the 3.50 gears.

Let me say again 6900 rpm in 4th gear will drop to 4400 rpm in 5th. This sucks, sucks, sucks, and I hate it, hate it, hate it.

I have read where some fellows with big blocks, like the .64 for highway cruising. I suspect mild cams, too.

All I can say is, if you want a .64 for highway cruising, then buy a 6 speed.