Gear swap time: Turns out I have 3.42 gears
 

Well I recently found out that I have 3.42 rear axle gears, throught I had 3.08 gears and a factory posi differential all along.
Here:
http://ecomodder.com/forum/showthrea...new-26556.html

It kind of makes sense, I always thought I should have been getting better highway milage.

This means there is more room to pickup some MPGs.
It also means that my 8.5inch GM 10 bolt has a serries 3 gear carrier. This allows me to use the Eaton 10 bolt posi that I picked up may years ago in my axle.
Serries 3 carriers have a wider selection of gear ratios and the gear sets are cheapest of the all the carriers.
(if I had a 3.08 rear it would be a serries 1 carrier and would have fewer swap choices, but then it would already have the 3.08s I want with a posi from the factory)

I am thinking about installing 3.08s or 3.23s.
I'm leaning towards 3.08s.
What do you think?

What tranny do you have again?

TH700R4, with a throttle value cable adjusted for firmer shifts (broke original transmission mount upon completion of adjustment) and a massivily over kill transmission filter/cooler setup.

Permalinks to my transcooler/filter build:
Remote 8 pass transcooler, with fan and sheet metal pebble guard:
Question: How to hook up a transmission cooler - Page 3 - Diesel Place : Chevrolet and GMC Diesel Truck Forums

Installed remote transcooler, plus you can see my nappa #1422 hydraulic fluid filter that I am using as my transmission filter. Its been installed since 2006 but I disconnected (bypassed) it in 2008 and I just recently replaced the filter, cleaned the filter head and put it back into service.
http://www.dieselplace.com/forum/63-...ml#post4957393

Everything else, front mount cooler, cooler loop, transmission fluid thermostat, transmission temperature gauge, remote cooler fan switch.
Question: How to hook up a transmission cooler - Page 4 - Diesel Place : Chevrolet and GMC Diesel Truck Forums
(lots of pictures)

The 700 serries has a lockup converter, 1:1 third gear, 0.71 OD and I have 31 inch tires.

My brother has an f150 and had a 2.64 or something like that but he burned it up so I gave him a 3.08. Mileage is about the same. The 2.64 was hard to get going without killing it. The 3.08 is better, but I like my 3.55 for all around work power.

I ran the numbers on a TH700 transmission, 31'' tires with 2.56 gears (GM 10 bolts tallest production ratio) a few years ago, it wouldn't work very well.

I don't think it would be a problem with the low end power in the diesel. I had an 85 caprice coupe with 2.56 gears and a 700R4. I put the springs from the police package on and had to run 225/75/15s to fill in the gap. That's 28.3" tire. The first gear in the 700r4 will get anything moving quickly and on the highway it would cruise with almost no throttle. You would be turning 100 rpm less than I was at 55 mph with your tire difference.

Overdrive would be a better way to go, IMO. That much mass -- loaded for work and/or towing -- is going to suffer disproportionately in acceleration events with a 3.08, even with a hotrodded motor.

An NV-4500 trans would interest me more than the proposed gear change.

An NV4500 would be pretty cool but they are kind of hard to find and expensive.
Plus I don't really want to do a flex plate to fly wheel swap, my rotating assembly is machine shop balanced.

I know of the gearvender over drive units but at $2k its way out of my price range.

I don't think it would be a problem with the low end power in the diesel. I had an 85 caprice coupe with 2.56 gears and a 700R4.
I agree the numbers work with a diesel and your trying for max mpg not towing.
A simple shift kit and perhaps redline clutched 700 is a very nice match for your issue. but beware the speed issue.
Should roll on well with grunt powered diesel idleing at 50 mph.

My best friend has a half ton GM 10-bold rear end up under his 1982 chevy pickup truck bed trailer and it has 2.56 gears.

Only problem is he lives in Virginia.

I still expect to do some towing. Just not enough towing to justify having 3.42 gears all the time.
I also drive off road but not enough to justify 4wd or 3.42 gears (that's why I have a 2wd). I would like to keep AT tires on the back at least and get the eaton posi in there.