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Old 05-15-2018, 01:15 PM   #1
s10mk
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Considering re-gearing my c20

I'm considering re-gearing my 72 c20( 350/350 dana 60) in an effort to make it a little more Comfortable to drive on the highway. It currently has 4.10 gears, and I would switch to a 3.55. Using an rpm calculator, I figured that currently at 60 mph I'm turning 2800 rpm. If I switched to 3.55s, I would be able to drive 69 mph at the same 2800 rpm. What are your thoughts?

I also considered swapping transmissions to a 700r4/200-4r, but my current th350 shifts really well, and i would rather just leave it in place.
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Old 05-15-2018, 07:58 PM   #2
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Re: Considering re-gearing my c20

Ive been thinking about the same thing. My set up in my C20 is 396/TH400 with 4.11s in the rear. It's doing 3000RPM at 60 MPH...

Ive considered 3.55s, the 700R4, or even the Gear Vendors auxiliary overdrive, but thats getting really spendy. Supposedly there's 5 speeds out there that will work...
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Old 05-15-2018, 08:15 PM   #3
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Re: Considering re-gearing my c20

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Originally Posted by s10mk View Post
I'my current th350 shifts really well, and i would rather just leave it in place.
in that case, yes, I agree with slingus - you may want to consider a Gear Vendors over/underdrive. Bolts to the back of your TH350, is bulletproof, eliminates the worry some have about setting line pressure/TV cable properly in a 700R4, and is completely reversible to stock arrangement (especially if you keep you current driveshaft intact). Not inexpensive - plan on about $3K, but not much more than a rebuilt (well) 700R4, TV Made EZ kit, possible torque converter, etc. I believe it would also eliminate your carrier bearing on a longbed. Requires a shortened driveshaft, but recommend just have a custom one made of the correct length and retain the original for that reversibility mentioned above in case you ever sell the truck.

If you just swap gears for a 3.54 in a D60, you will help your high rpm highway driving, but you lose low end torque multiplication in 1st gear by the same factor. That doesn't happen with an overdrive or a GV unit. GV unit can also be used as a gear splitter (makes your TH350 a 6 speed) or just as an overdrive 4th gear, depending on how you set it up.

Just a thought for consideration.
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Old 05-15-2018, 08:17 PM   #4
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Re: Considering re-gearing my c20

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Originally Posted by Slingus View Post
Ive been thinking about the same thing. My set up in my C20 is 396/TH400 with 4.11s in the rear. It's doing 3000RPM at 60 MPH...

Ive considered 3.55s, the 700R4, or even the Gear Vendors auxiliary overdrive, but thats getting really spendy. Supposedly there's 5 speeds out there that will work...
Yes, NV4500 is the preferred option for a manual 5-speed overdrive for your big block - you can search on here for several very good threads on how to do the conversion.
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Old 05-16-2018, 04:19 PM   #5
s10mk
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Re: Considering re-gearing my c20

The gear vendors is a good option, but at $3K, its just out of my price range. I would just as soon pocket that money and save it to find a nice k20.

But fortunately, I build my own transmissions, so I could get a used/ junkyard 200-4r, converter, and a rebuild kit for relatively cheap. So I do have that option if I decide against a 3.55 gear swap
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Old 05-17-2018, 10:08 AM   #6
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Re: Considering re-gearing my c20

I'm actually doing the same thing right now. Just picked up a 2004R trans and looks to be pretty much a direct bolt in. Waiting for my TV bracket, and speedo gears to arrive and I'll get it installed. I'm going to post a writeup when I get started.
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Old 05-17-2018, 07:57 PM   #7
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Re: Considering re-gearing my c20

Hunt up a modern 14 bolt and you can do alot better ratio wise
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Old 05-17-2018, 09:44 PM   #8
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Re: Considering re-gearing my c20

I myself have been thinking about doing the same, 3.73 though and a gear vendors unit. I want better than 350 hp and 425 on the torque to push 4100 lbs.
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Old 05-18-2018, 12:36 PM   #9
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Re: Considering re-gearing my c20

I just had a Dana 60 rear put into my '72 K20 with 3.54 gears. At the same time I had the Dana 44 front changed to 3.54. Wow! What a nice combination! I have a pretty tall tire (265/75/16) and it rolls right with highway traffic and feels great. BTW, at 70 mph, my speedo reads 57 (need to update my speedo gearing). I had contemplated the Gear Vendor solution but am SUPER pleased with the 3.54 gears. As an added bonus, the rear swap finally resolved a vibration issue the truck has had.
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Old 05-18-2018, 07:28 PM   #10
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Re: Considering re-gearing my c20

I didn’t know you could get a 3.54 for a d44 - that’s good to know. Yes, I agree, that sounds like a great combo.

For the OP, considering your trans skills and parts availability, I’d lean toward the trans option over the rear swap only, you increase 1st gear torque multiplication (the oft-undiscussed advantage of an o/d trans) and reduce highway rpm.
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Old 05-18-2018, 09:39 PM   #11
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Re: Considering re-gearing my c20

I swapped the 4.56 gears in my Dana 60 for 3.54 and my six does well with it. That's with 285/70r17 tires. It could use closer gear spacing on the hills, when carrying a load than the SM465 provides, but it's far better than dealing with the 4.56 gears.

Be sure and check axle spline count on your dana if you are changing carrier. You don't, but I had to. I got my 3.54 gears with posi carrier out of a junkyard Dodge for $50. Bearings, shims, seals and grease added an additional 3 times that amount.

Dodge pinion splines are different and they use smaller U-joints. I paid $20 for a Dodge yoke made for the larger U-joints. I don't know anything about fnord dana.
And as you probably already know, Dana gears aren't the easiest to set up.

I'm happy with the 3.54 gears, but if I ever went to 4 spd OD Auto or a 5 spd, it would be better with 3.73 gears.
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Old 05-19-2018, 02:43 PM   #12
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Re: Considering re-gearing my c20

I was told by Extreme Automatics that 2004r's are great transmissions but will not tolerate towing.
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Old 05-20-2018, 07:21 PM   #13
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Re: Considering re-gearing my c20

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I was told by Extreme Automatics that 2004r's are great transmissions but will not tolerate towing.
Correct. GM never installed a 200-4R in a truck, let alone a 6700 gvwr for good reason. Remember, the 200 is based on a TH200, the 700-R4 on a TH350, the 4L80 on a TH400.

The 700-R4 is mostly same as 4L60, although 4L60 has stiffer apply plates and more friction discs. This was the reason the 700 got a bad rep. Towing would cause 3/4 disc stack to slip and burn. The 4L65 is same as 4L60, but with 5 pinion planetaries and improved sun shell with thrust bearing.

So, might as well put in a 700 with 4L60 disc stack and slightly higher apply/system pressure. With a cooler, this would be equivalent to your TH350.

If you want tow without fear in overdrive, 4L80 is preferred.
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