I had my original TH125C swapped out for a 4T60. While I love the lower rpms, it is a bit of a dog now with the stock 1988 GT 2.8L.
The ID on the trans was completely wash away. So that make identification a little tough.

Using http://www.differentials.co...ear-ratio-calculator to calculate what it might be it looks to be a 2.84.
My rpms at 70mph are approx 1870 and I have the stock size tires...diameter 25 1/4". I verified the rpms by usin my scan tool hooked to the ALDL and monitored it in live mode.

My highway mileage is 27mpg, which I love, but the acceleration at highway speeds is what annoys me.

Living in Arizona, we have mountains everywhere and the uphill climbs are a bit of a challenge.

What is the complexity of changing the final drive ratio on my 4T60 / TH440? Are there any great how-to articles?

I still have the original TH125C and axels if anything from it could be used in teh 4T60. I know they are completely different beasts, but GM does like to use as many parts across the different models to keep costs down.

Here's more than I ever wanted to know about it,

http://www.gmtuners.com/tech/440_4T60E_4T65E.htm

There are ways to change the places in the gears that it down shifts if that is what you want so that it downshifts earlier when you reach a hill. there are limits to what will be helpful and what will affect MPG.


Steve

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Technology is great when it works,
and one big pain in the ass when it doesn't



Detroit iron rules all the rest are just toys.

[This message has been edited by 84fiero123 (edited 07-27-2014).]

Look here too: http://web.archive.org/web/...s.com/tech/440-4T60/

When I had the 4T60 in my Indy, I had the 3.33 final drive (35/35 gears). Didn't like it.

I dropped the trans and changed it to 3.73 (33.37 gears). Hated it worse.
It felt like a 5-speed with 4th gear missing. Swapped in a Getrag 5-speed afterwards.

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What did you not like about the 3.33 final drive?

quote Originally posted by fierosound: Look here too: http://web.archive.org/web/...s.com/tech/440-4T60/ When I had the 4T60 in my Indy, I had the 3.33 final drive (35/35 gears). Didn't like it. I dropped the trans and changed it to 3.73 (33.37 gears). Hated it worse. It felt like a 5-speed with 4th gear missing. Swapped in a Getrag 5-speed afterwards.

If the mountain areas are your concern, why not treat it like a stick and drop it into 3rd or 2nd gear when going up a hill? I'm planning on doing this same swap, but it is very flat here in Va. Beach, so hills are not a concern for me.

Jim

quote Originally posted by My88RedGT: What did you not like about the 3.33 final drive?

It didn't go into 4th until 60mph. Changing it to 3.73 made it worse. You'd be at a speed where you were too slow for 4th, but the engine had high revs in 3rd. Looking back, the 3.33 is a better combination.

I changed to a Getrag 5-speed because the auto felt like a 5-speed with 4th gear missing.

[This message has been edited by fierosound (edited 07-28-2014).]

quote Originally posted by My88RedGT: Living in Arizona, we have mountains everywhere and the uphill climbs are a bit of a challenge. What is the complexity of changing the final drive ratio on my 4T60 / TH440? Are there any great how-to articles?

Changing the final drive ratio in the 440T4/4T60 transmissions requires complete transmission disassembly. You are not going to be able to do this in the car. An ATSG Transmission Rebuilding manual explains the process and tools needed to do it.

The stock differential ratios available that work in the 440T4/4T60 are 2.84, 3.06, and 3.33. Common chain sprocket ratios available are 33/37, 37/33, and 35/35. Based on your calculations, it sounds like you have 35/35 chain sprockets and a 2.84 ratio diff.

I wouldn't mess with changing the chain sprockets and would instead recommend you just change the diff. Since you drive in the mountains, a 3.33 ratio diff would probably be your best choice. It will perform better than a 125-C that came with a 3.33 ratio diff and it will still let your engine run at lower RPMs in OD/4th gear than a 125-C transmission that has a 2.84 ratio diff, so it will still get better gas mileage. Swapping in a 3.06 ratio diff would make a small improvement in acceleration over what you have now but it may not be enough for the mountain roads. Keep in mind you'll probably also need to change the shift governor to match whatever new gear ratio you install. I'm not certain whether or not valve body changes will be required to maintain proper part and full throttle shift points.

-ryan

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[This message has been edited by Darth Fiero (edited 07-28-2014).]