1971 GMC C2500 rear end questions

[Baime]

Originally Posted by Ironangel

Baime, thats a Dana 60 in your pic, KEEP IT! Dont let anyone who dont know what it is touch it! There is no such thing as a "T400R4"...Need to determine what gearing is in it now by pulling the cover and getting the numbers off the ring gear. You will also be able to determine what carrier you have at the same time. The Dana 60 only goes as high as 3:55 and as low as 7:17 with the carrier break at 4:10/4:56. Nows the time to ask yourself what exactly are your plans for your 3/4 ton Jimmy? Are you pulling heavy trailers and hauling heavy loads? Do you have a $3K to invest in a gear set, a posi or locker, and a questionable 700r4? Not trying to rain on your parade but transforming a 71 3/4 ton GMC into gas friendly daily driver is an expensive endeavor. The least expensive way out is a new 3:55 gear set and taller 16" tires and wheels...My daily driver has 3:73 gears turning 31" 275/70R-16's with a built 350 Turbo to pull a tandem trailer with another 4000lb truck on it. If mileage is paramount, then you cant afford to drive these old trucks..."Just Sayin"

IronAngel,
I agree, I have to make this decision based on how I want to use my truck. I just really like my truck, and started to think that changing the rear end gearing and adding positraction would be a nice update/improvement. It isn't a necessary change. It obviously is geared more for pulling and hauling, than going 75mph on the highway.

How do you like your setup with the 350 Turbo, and the 3:73 gears?
I believe I have the 350 Turbo with 4:11 gears. It could be 4:10. Any idea what rpms you engine is running at going 70mph with the 3:73 gears?


I mostly make highway trips up to my cabin a hundred miles away, hauling what I need in the back, and I pull my boat a few hundred miles a year. I also enjoy just driving it around town in the non-winter months.

Thanks,
JP

[Baime]

An update on my efforts to update my truck gearing:
I went out to my cousin's pole barn last night to look at the old rear end he had in his shop. He has a 14 bolt rear end for a 3/4 ton GM, but said I don't want that if my existing rear end is a Dana 60. We confirmed that it was a Dana 60. He was surprised too.

My cousin is one of the best mechanics that I know. He is in a 4-wheeler club, and is building up 4-wheel trucks in his barn all of the time, so I got his opinion while I was out there.

His comments:
- He would just leave it as is. Unless I have plans to drive it all over the country, he didn't think the cost of the updates justified making the changes. (This sounds too logical. He obviously doesn't like my truck as much as I like it.)
- He believes the newer 400-R4 and 200-R4 overdrive transmissions are not as strong as the original T350 and T400 transmissions. He recommended keeping my existing T350 transmission, and keeping the Dana 60 rear end.
- He thought I should find a used Dana 60 with the lower gears already in it, and change out the rear end, for the most cost effective way to get higher gears in the rear end.
- Otherwise he would update the existing Dana 60. He also knows "a guy" who updates a lot of rear differential in his shop. He called his guy for ballpark pricing. He was booked until spring, and said he would do it for about $1000 without positraction, and add another $500 if I want positraction.

Note: I'm now getting pricing from my friends "guy" who previously recommended a new rear end, because they both assumed that I had the Eaton 5200.

Thanks again for your comments. JP

[tdangle]

Originally Posted by Baime

Tdangle,
Thanks for your comments.
Your setup sounds nice. Is the 2004r OD automatic transmission different from the T400R4 transmission? Is there an advantage to the 2004r over the T400R4?

Is that a difficult conversion from the T350 auto?
- Shorten the drive shaft.
- New starter and flywheel?
- Does the shifting linkage line up, or is there some type of conversion kit?

Take care,

JP

Not sure what a t400r4 is. Maybe the same just spelled out differently. I choose mine because I had a TH350 and it is the same length so I didn't have to shorten the front driveshaft. Was a direct bolt in. Moved the cross member back to the TH400 position, got a bracket for the TV cable that matched up to my quadrajet and installed a lockup switch setup so the torque converter locks up in 4th automatically. I did have to mess with the shift linkage a bit. Same starter, flywheel just had to get metric bolts for the torque converter. Some say it may not be the strongest trans, but I am not towing really heavy trailers or racing it.

[StingRay]

A 14 bolt full floater or even a 9.5 semi float are very good, plentiful and cheap. Parts are everywhere. If you are on a budget do a swap. Hang onto the Dana.