Rear gear swap 3.90 to 3.38 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

northerngmc · 03-14-2011, 11:32 AM

Has anyone here swaped the ring gear and pinion from 3.90 to 3.38 in the original rear end?

Just wonder how complicated the swap is?

And also wondering how well my original 235 will go with the 3.38 rear gear.

Hoping to get better highway speed out of my 59 GMC with the original 235.

Here is the link to the parts I am looking at.
http://www.tomsdifferentials.com/catalog.asp?pg=15

**WIDESIDE72** · 03-14-2011, 11:42 AM

I am not familiar with the gear ratios that are originally available in 59's. Was the 3.38 a ratio available from the factory? Are you dealing with the stock rear axle? IF SO, Your 59 has a drop out center section, correct? If so, and you had the whole center section with the 3.38 ratio, it should be an afternoon swap. If you don't my advice would be to find another center section and have it set up with the gears you want, that way you could swap back to the 3.90 ratio later if you change your mind. It would cost a little more money to purchase another cener section, but I think it would be worth it.

**WIDESIDE72** · 03-14-2011, 11:44 AM

In my dad's 71 GMC w/ a 250 6 cyl, we swapped the whole rear axle with a 3.73 rear for a 3.07 posi out of a big block truck. You have to rev it a lttle more to get going from a dead stop, but it goes down the highway like a modern truck now. He doesn't plan to tow anything with it.

northerngmc · 03-14-2011, 11:50 AM

I believe the 3.38 was a factory option.

Yes I have the factory rear end in my truck.

Not sure what you mean by "drop out center", but I know my axles are held in with C clips.

I have another complete rear so finding a second center section would not be a problem.

This truck is only driven for show and will not see towing duty or hauling duty.

**WIDESIDE72** · 03-14-2011, 10:12 PM

By drop out center, I mean that the axles can be pulled and the bolts removed from the front of the center section, and the whole center section can be removed. THis is what my 55 has. What is convenient aboutthis style of rear end is the center can be puleed and set on a work bench to set up the gears, unlike our 71, where all of the work has to be done under the truck, or the whole rear axle assembly removed from the truck.

northerngmc · 03-14-2011, 10:23 PM

So when you unbolt the front of the carrier does the pinion come out as well?

**mr48chev** · 03-14-2011, 11:17 PM

Quote:

Originally Posted by northerngmc

So when you unbolt the front of the carrier does the pinion come out as well?

If you have the drop out center section it all comes out in one piece. It also makes them a bit easier to work on, at lest for me as you can put it up on the bench to work on it. Or haul just the third member to the person or shop you are going to have do the work.

I got lazy instead of digging up a photo I just found an Ebay listing for a third member as an example.
http://cgi.ebay.com/ebaymotors/46-56...Q5fAccessories

beanyard · 03-15-2011, 12:38 AM

I went through all this with my truck. If wou want the lo down send me your phone number and we can talk in great detail. In summary, the parts are expensive and looking back I would have done a late model rear end with five lugs for a lot less doe. I went with the factory 3.90 because I have the two speed automatic overdrive option.

northerngmc · 03-15-2011, 11:27 AM

Quote:

Originally Posted by mr48chev

If you have the drop out center section it all comes out in one piece. It also makes them a bit easier to work on, at lest for me as you can put it up on the bench to work on it. Or haul just the third member to the person or shop you are going to have do the work.

I got lazy instead of digging up a photo I just found an Ebay listing for a third member as an example.
http://cgi.ebay.com/ebaymotors/46-56...Q5fAccessories

Yes a picture really helped.

northerngmc · 03-15-2011, 11:28 AM

Looking at the prices at different vendors there is quite a difference.

Tom' Differentials
Rear gear and pinion $265
Installation Kit $115

Total $380

Classic Parts

Rear gear and pinion $400
Installation Kit $200

Total $600

Anybody have experience with Tom's Differential?

magicoolbus · 03-15-2011, 12:18 PM

Be sure to check out Patrick's in Casa Grande AZ. He has been doing this swap for eons. http://www.patricksantiquecars.com/chevy_parts.html

399 for his ring/pinon and install kit.

03-14-2011, 11:32 AM	#1
northerngmc Registered User Join Date: Jan 2004 Location: Whitehorse YK Canada Posts: 2,651	Rear gear swap 3.90 to 3.38 Has anyone here swaped the ring gear and pinion from 3.90 to 3.38 in the original rear end? Just wonder how complicated the swap is? And also wondering how well my original 235 will go with the 3.38 rear gear. Hoping to get better highway speed out of my 59 GMC with the original 235. Here is the link to the parts I am looking at. http://www.tomsdifferentials.com/catalog.asp?pg=15 __________________ Blaine 59 GMC 9314 a.k.a. Whoopee 68 C20 50th Anniversary a.k.a. The Rat http://www.flickr.com/photos/northerngmc/ http://www.members.tripod.com/P-wee/index_032.htm 1.61803 is one H of a lot better than 3.14159, no really it is!

03-14-2011, 11:42 AM	#2
WIDESIDE72 Senior Member Join Date: Aug 2008 Location: Cedar Park, Texas Posts: 7,500	Re: Rear gear swap 3.90 to 3.38 I am not familiar with the gear ratios that are originally available in 59's. Was the 3.38 a ratio available from the factory? Are you dealing with the stock rear axle? IF SO, Your 59 has a drop out center section, correct? If so, and you had the whole center section with the 3.38 ratio, it should be an afternoon swap. If you don't my advice would be to find another center section and have it set up with the gears you want, that way you could swap back to the 3.90 ratio later if you change your mind. It would cost a little more money to purchase another cener section, but I think it would be worth it.

03-14-2011, 11:44 AM	#3
WIDESIDE72 Senior Member Join Date: Aug 2008 Location: Cedar Park, Texas Posts: 7,500	Re: Rear gear swap 3.90 to 3.38 In my dad's 71 GMC w/ a 250 6 cyl, we swapped the whole rear axle with a 3.73 rear for a 3.07 posi out of a big block truck. You have to rev it a lttle more to get going from a dead stop, but it goes down the highway like a modern truck now. He doesn't plan to tow anything with it.

03-14-2011, 11:50 AM	#4
northerngmc Registered User Join Date: Jan 2004 Location: Whitehorse YK Canada Posts: 2,651	Re: Rear gear swap 3.90 to 3.38 I believe the 3.38 was a factory option. Yes I have the factory rear end in my truck. Not sure what you mean by "drop out center", but I know my axles are held in with C clips. I have another complete rear so finding a second center section would not be a problem. This truck is only driven for show and will not see towing duty or hauling duty. __________________ Blaine 59 GMC 9314 a.k.a. Whoopee 68 C20 50th Anniversary a.k.a. The Rat http://www.flickr.com/photos/northerngmc/ http://www.members.tripod.com/P-wee/index_032.htm 1.61803 is one H of a lot better than 3.14159, no really it is!

03-14-2011, 10:12 PM	#5
WIDESIDE72 Senior Member Join Date: Aug 2008 Location: Cedar Park, Texas Posts: 7,500	Re: Rear gear swap 3.90 to 3.38 By drop out center, I mean that the axles can be pulled and the bolts removed from the front of the center section, and the whole center section can be removed. THis is what my 55 has. What is convenient aboutthis style of rear end is the center can be puleed and set on a work bench to set up the gears, unlike our 71, where all of the work has to be done under the truck, or the whole rear axle assembly removed from the truck.

03-14-2011, 10:23 PM	#6
northerngmc Registered User Join Date: Jan 2004 Location: Whitehorse YK Canada Posts: 2,651	Re: Rear gear swap 3.90 to 3.38 So when you unbolt the front of the carrier does the pinion come out as well? __________________ Blaine 59 GMC 9314 a.k.a. Whoopee 68 C20 50th Anniversary a.k.a. The Rat http://www.flickr.com/photos/northerngmc/ http://www.members.tripod.com/P-wee/index_032.htm 1.61803 is one H of a lot better than 3.14159, no really it is!

03-15-2011, 12:38 AM	#8
beanyard Registered User Join Date: Mar 2011 Location: Idaho Falls, Idaho Posts: 247	Re: Rear gear swap 3.90 to 3.38 I went through all this with my truck. If wou want the lo down send me your phone number and we can talk in great detail. In summary, the parts are expensive and looking back I would have done a late model rear end with five lugs for a lot less doe. I went with the factory 3.90 because I have the two speed automatic overdrive option.

03-15-2011, 11:28 AM	#10
northerngmc Registered User Join Date: Jan 2004 Location: Whitehorse YK Canada Posts: 2,651	Re: Rear gear swap 3.90 to 3.38 Looking at the prices at different vendors there is quite a difference. Tom' Differentials Rear gear and pinion $265 Installation Kit $115 Total $380 Classic Parts Rear gear and pinion $400 Installation Kit $200 Total $600 Anybody have experience with Tom's Differential? __________________ Blaine 59 GMC 9314 a.k.a. Whoopee 68 C20 50th Anniversary a.k.a. The Rat http://www.flickr.com/photos/northerngmc/ http://www.members.tripod.com/P-wee/index_032.htm 1.61803 is one H of a lot better than 3.14159, no really it is! Last edited by northerngmc; 03-15-2011 at 11:32 AM. Reason: Wasn't finished typing DOH!

03-15-2011, 12:18 PM	#11
magicoolbus Registered User Join Date: Jul 2010 Location: Mesa, AZ Posts: 513	Re: Rear gear swap 3.90 to 3.38 Be sure to check out Patrick's in Casa Grande AZ. He has been doing this swap for eons. http://www.patricksantiquecars.com/chevy_parts.html 399 for his ring/pinon and install kit. __________________ JB from AZ 1969 Chevy Sub K10 1957 Chevy 3200 1962 VW Beetle 1957 Willys CJ5



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