2 speed rear end, 5 speed info needed

[HEI451]

I am in the planning stages of doing a 2 speed rear end for my 1964 Chevy C30 1 ton dually flatbed. I have the following questions for the board:

I presume the 2 speed rears were used on C40, 50, 60 series trucks?

Were they 1:1 and an over drive, or, under drive and 1:1?

The whole thing is to reduce the rpms of the big block 305E V6 I have in the truck on the highway. As is now, I have a very low gear single speed rear, and granny low 4 speed, with 1:1 top gear.

The next plan is to find a 5 speed with both granny low, and over drive top gear ratios, don't know if that exists, but asking might get answers I need.

The whole thing is to gain the best of two worlds, granny low torque pulling ability, and reduced rpm highway cruising at speed.

Thanks for any info.

[j.kirn]

My 64 c-60 has a new process 5 speed and 2 speed rear

[j.kirn]

Your c-30 probably has 457 or 512 gears in it

[padresag]

both clark and np used a 5 speed o/drive. I used a np 5speed o/d in my 64 behind a detroit for 18 years.o/d was good. the only d/fall id the 3-4 shift is really wide in ratio and you will drop to many rpm when you may need them
check this page out http://www.6066gmcguy.org/SM420.htm
ron

[Captainfab]

I will have to check to verify but I believe the 2 speed rear in my '63 C60 is 6.72 & 7.17. I don't know what size wheels you will be able to bolt up other than 19.5's or 20's. You might be able to put some lo-pro 19.5's on there and get them under your C30. But then your effective gear ratio will be even lower, than with the 9.00-20's on my C60.

I would think that a 4.10 rear gear and a NV4500 should get you where you want to be with a tall enough tire.

[HEI451]

The other option would be to use what a friend gave me, a Hone inline over drive unit. I have the unit available if I want it, but it isn't one that bolts to the rear of the stock trans, so, I'd either have to make an adapter to bolt it up to the trans, or, run two shortened drive shafts into and out of the over drive unit, and mount the unit to the chassis.

I am tempted to just go "borrow" the 2 speed rear off my neighbor's Dodge dump truck we built a couple of years ago, he never drives it.

Thanks for the answers, keep 'em coming, please.

I just found a 1965 3/4 ton GMC 4 wheel drive 6 lug pickup sitting in a field with its hood up. Has been there for at least a decade, no radiator, everything else is there, though. I think I can get it for the princely sum of "hey, drag that junker outta here". Hope so.