3:38 rearend speedo adapter - The 1947 - Present Chevrolet & GMC Truck Message Board Network

northerngmc · 12-19-2011, 06:24 PM

I picked up a 3:38 rear gear conversion kit for the original diff in my 59 GMC.

The dealer was unable to get the speedo adapter for this conversion. Has anyone who has changed an 55T diff to higher gears changed the speedo adapter to ensure the speedo is accurate?

The truck has the original 3 speed manual trans.

I am wondering how this adapter works, is it external to the trans?
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solidaxel · 12-19-2011, 06:33 PM

Does your transmission not have a removeable Speedo gear on the tailshaft?

northerngmc · 12-19-2011, 06:55 PM

Not sure, truck is stored away for the winter..... I kind of thought it did but was unsure.

Anybody know of a vendor that might sell the new gear?
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northerngmc · 05-10-2012, 01:14 PM

ttt for spring..... truck comes out in 8 weeks!

**G&R's57GMC** · 05-10-2012, 04:25 PM

You'll need a ratio adapter.

Check out http://motors.shop.ebay.com/i.html?_...+ratio+adapter

Or Google 'speedometer ratio adapter'

**mr48chev** · 05-10-2012, 05:31 PM

G&R's57GMC beat me to it. Those little boxes have what looks like a quick change gear set in them that you swap gears around in to calibrate the speedometer with.

If you have access to a hand held gps take a drive down the road and at something like 50 or 60 mph on the speedo record both speeds (GPS and speedometer) and figure out the percentage of difference and that is what the gears in the box would have to be set for.
Most any big truck shop (semi) should have those boxes and the gears and a lot of 4x4's run them. I've pulled several of 4x4 trucks in wrecking yards over the years but don't think I have one left.

These guys show most all of the removable speedometer drive and driven gears and give instruction on how to calculate what you need. http://www.transmissioncenter.net/sp...n_______va.htm

northerngmc · 05-10-2012, 05:32 PM

I believe 3:38 was a factory option, so I am hoping to just change the speedo gear on the trans.

Coupeguy2001 · 05-22-2012, 09:17 PM

I ran 3.38 gears with my 57 with 1:1 tail shaft speed and 275/60/15 and the speedo was only off 5 mph at 70 mph according to my GPS. It read higher than I was going.
Just for a reference...........
You need to go to tire rack dot com and check out that tire size on the specs, and it will tell you the diameter of the tire and the circumference. then look up 670-15 on coker or elsewhere for the same specs. That will tell you what you need to adjust your tire size to get speedo dialed in if the adapter is only close.

northerngmc · 05-22-2012, 11:14 PM

Thank you very much for the information.
Posted via Mobile Device

12-19-2011, 06:24 PM	#1
northerngmc Registered User Join Date: Jan 2004 Location: Whitehorse YK Canada Posts: 2,651	3:38 rearend speedo adapter I picked up a 3:38 rear gear conversion kit for the original diff in my 59 GMC. The dealer was unable to get the speedo adapter for this conversion. Has anyone who has changed an 55T diff to higher gears changed the speedo adapter to ensure the speedo is accurate? The truck has the original 3 speed manual trans. I am wondering how this adapter works, is it external to the trans? Posted via Mobile Device __________________ 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!

12-19-2011, 06:33 PM	#2
solidaxel Registered User Join Date: Dec 2002 Location: Cactus Patch So. Az Posts: 4,749	Re: 3:38 rearend speedo adapter Does your transmission not have a removeable Speedo gear on the tailshaft? __________________ 53 TuTone Extended Cab 350 4-Spd 3:08 (SOLD) 53 Chevy Moldy pearl green ZZ-4 4L60E 9" 3:25 55 GMC 1st Black Mll (ZZ4) ZZ6 TKO 600 5 sp 3:73 62 Solidaxle Corvette Roman Red (327 340hp 4spd 3:36) C4 & C5 suspension tube chassis LS 3 4L70E 65 Corvette Coupe 327 350hp 4spd 4:11 78 Black Silverado SWB (350/350) 5.3 & 4L60E 3:42 2000 S-Type 3.0 (wife cruiser) 2003 GMC SCSB 5.3 4L60E 3:42

12-19-2011, 06:55 PM	#3
northerngmc Registered User Join Date: Jan 2004 Location: Whitehorse YK Canada Posts: 2,651	Re: 3:38 rearend speedo adapter Not sure, truck is stored away for the winter..... I kind of thought it did but was unsure. Anybody know of a vendor that might sell the new gear? Posted via Mobile Device __________________ 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!

05-10-2012, 01:14 PM	#4
northerngmc Registered User Join Date: Jan 2004 Location: Whitehorse YK Canada Posts: 2,651	Re: 3:38 rearend speedo adapter ttt for spring..... truck comes out in 8 weeks! __________________ 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!

05-10-2012, 04:25 PM	#5
G&R's57GMC Senior Member Join Date: Aug 2010 Location: Shasta Lake, CA. Posts: 1,624	Re: 3:38 rearend speedo adapter You'll need a ratio adapter. Check out http://motors.shop.ebay.com/i.html?_...+ratio+adapter Or Google 'speedometer ratio adapter' Attached Images

05-10-2012, 05:31 PM	#6
mr48chev Registered User Join Date: Jul 2007 Location: Toppenish, WA Posts: 15,718	Re: 3:38 rearend speedo adapter G&R's57GMC beat me to it. Those little boxes have what looks like a quick change gear set in them that you swap gears around in to calibrate the speedometer with. If you have access to a hand held gps take a drive down the road and at something like 50 or 60 mph on the speedo record both speeds (GPS and speedometer) and figure out the percentage of difference and that is what the gears in the box would have to be set for. Most any big truck shop (semi) should have those boxes and the gears and a lot of 4x4's run them. I've pulled several of 4x4 trucks in wrecking yards over the years but don't think I have one left. These guys show most all of the removable speedometer drive and driven gears and give instruction on how to calculate what you need. http://www.transmissioncenter.net/sp...n_______va.htm

05-10-2012, 05:32 PM	#7
northerngmc Registered User Join Date: Jan 2004 Location: Whitehorse YK Canada Posts: 2,651	Re: 3:38 rearend speedo adapter I believe 3:38 was a factory option, so I am hoping to just change the speedo gear on the trans. __________________ 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!

05-22-2012, 09:17 PM	#8
Coupeguy2001 Registered User Join Date: Aug 2011 Location: phoenix az Posts: 723	Re: 3:38 rearend speedo adapter I ran 3.38 gears with my 57 with 1:1 tail shaft speed and 275/60/15 and the speedo was only off 5 mph at 70 mph according to my GPS. It read higher than I was going. Just for a reference........... You need to go to tire rack dot com and check out that tire size on the specs, and it will tell you the diameter of the tire and the circumference. then look up 670-15 on coker or elsewhere for the same specs. That will tell you what you need to adjust your tire size to get speedo dialed in if the adapter is only close.

05-22-2012, 11:14 PM	#9
northerngmc Registered User Join Date: Jan 2004 Location: Whitehorse YK Canada Posts: 2,651	Re: 3:38 rearend speedo adapter Thank you very much for the information. Posted via Mobile Device __________________ 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!



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