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Old 09-26-2010, 11:45 PM   #1
BigRed76
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Speedometer Gear

I need some help with speedo gears. This is on my '76 K10 that has an NP205 t-case. My truck currently has 31" tires on it and i will soon be putting 35's on it and i want my speedo to read right afterwards. I dont currently know the tooth count on the driven gear in the t-case, but there is a ratio adapter between the driven gear and the speedo cable. Do i just need to replace the ratio adapter? or do i need to keep the ratio adapter and replace the driven gear? And if i do need to just switch out the ratio adapter then how do i find out which one i need?
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1970 Chevrolet Custom Camper K20
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1972 Oldsmobile Cutlass Supreme, 455/TH400, 3.73 posi
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Old 09-27-2010, 10:34 AM   #2
old Rusty C10
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Re: Speedometer Gear

anyone or should we move this to transmissions?
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Old 09-27-2010, 10:50 AM   #3
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Re: Speedometer Gear

First thing you really need to do is pull the speedo housing from the trans and see what tooth count the gear is and which speedo housing you have. The housing will either be a 34-39 housing or a 40-45 housing (it's actually stamped on the housing). Then you will have use a speedo calculator to figure out what gear you will need to correct you speedo. I use the one at http://www.transmissioncenter.org/GM...eter_Gears.htm ....They are pretty good to deal with too.

Good Luck!

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Old 09-27-2010, 03:55 PM   #4
BigRed76
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Re: Speedometer Gear

I understand how it is with the automatics, but I'm just unsure of how it is with the manuals where the speedo gear is in the t-case.
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1970 Chevrolet Custom Camper K20
1971 GMC Super K2500, 12V/NV4500 swap in progress
1971 Chevrolet Custom C10
1972 Chevrolet Custom Deluxe C20, 5.3/4L60E
1972 Oldsmobile Cutlass Supreme, 455/TH400, 3.73 posi
2004 GMC Sierra 2500HD, LB7/Allison, CCSB
2005 GMC Sierra 1500, 5.3/4L60E, CCSB

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Old 09-27-2010, 06:26 PM   #5
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Re: Speedometer Gear

It's the same on all of them. It's just moved further back is all. Check out that website I posted, they have tons of tech info about this stuff.

Ed
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Old 09-28-2010, 12:43 AM   #6
BigRed76
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Re: Speedometer Gear

Should we move this to the drivetrain section?
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1970 Chevrolet Custom Camper K20
1971 GMC Super K2500, 12V/NV4500 swap in progress
1971 Chevrolet Custom C10
1972 Chevrolet Custom Deluxe C20, 5.3/4L60E
1972 Oldsmobile Cutlass Supreme, 455/TH400, 3.73 posi
2004 GMC Sierra 2500HD, LB7/Allison, CCSB
2005 GMC Sierra 1500, 5.3/4L60E, CCSB

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Old 09-28-2010, 10:28 PM   #7
BigRed76
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Re: Speedometer Gear

just givin it a bump...
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1970 Chevrolet Custom Camper K20
1971 GMC Super K2500, 12V/NV4500 swap in progress
1971 Chevrolet Custom C10
1972 Chevrolet Custom Deluxe C20, 5.3/4L60E
1972 Oldsmobile Cutlass Supreme, 455/TH400, 3.73 posi
2004 GMC Sierra 2500HD, LB7/Allison, CCSB
2005 GMC Sierra 1500, 5.3/4L60E, CCSB

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