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01-22-2008, 12:07 PM
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#1 |
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67-72 Addicted
Join Date: Oct 2007
Location: Guelph Ontario
Posts: 256
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Gear ratios?
I have a 71 gmc 1/2 ton with a 350 and a TH400, and i have no idea what gears are in the rear-end. I have a 3/4 ton book for 1970 but It dosnt show anything for the 1/2 ton rear-ends...
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01-22-2008, 12:14 PM
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#2 |
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its all about the +6 inches
Join Date: Jan 2001
Location: Hilliard Ohio
Posts: 2,690
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Re: Gear ratios?
what engine and tranny were in it originally?
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01-22-2008, 12:22 PM
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#3 |
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67-72 Addicted
Join Date: Oct 2007
Location: Guelph Ontario
Posts: 256
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Re: Gear ratios?
It has the origonal engine and trans, 350 and TH400
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01-22-2008, 12:23 PM
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#4 |
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Future TOTM Winner
Join Date: Aug 2006
Location: Bulverde, TX
Posts: 2,692
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Re: Gear ratios?
If you are trying to determine the gear ratio:
Option 1 Remove the diff cover and count the teeth on the gear and pinion. Divide the gear # by the pinion # (the ratio is also stamped on the gear if you can find it). Option 2 Jack up one rear tire, put the truck in neutral (obviously make sure the truck is secure and not going to roll), and rotate the other tire one full rotation. Count how many rotations the drive shaft makes. If the drive shaft makes about 3 3/4 full rotations, you probably have 3.73s. If it makes just over 3 rotations, you probably have 3.07s etc...you get the idea. Option 3 Just look on SPID in your glove box if its still there. 
__________________
72 Chevy CST K5 HIGHLANDER BLAZER Getting a 6.0,NV4500,NP205,14BFF (Currently laying all over the place in little pieces) 72 Chevy swb step-side "LS" 6.0 Here's a build thread of sorts 2002 HD2500 Crew Cab, 8.1L Allison 4x4 Daily Driver Check it out www.lsdyno.com Last edited by Critter; 01-22-2008 at 01:19 PM. |
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01-22-2008, 12:34 PM
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#5 |
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67-72 Addicted
Join Date: Oct 2007
Location: Guelph Ontario
Posts: 256
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Re: Gear ratios?
My truck is in storage over the winter and I wanted to see if I should change my gears when i add a 5 speed, and I know that you can find out the origonal gears if u know the engine and trans. But thanks anyway critter
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01-22-2008, 12:38 PM
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#6 |
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67-72 Addicted
Join Date: Oct 2007
Location: Guelph Ontario
Posts: 256
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Re: Gear ratios?
Whats the SPID? like the option sheet on the glove box door, i took a look at the one on my Longhorn but i dont see any gear ratios on it...
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01-22-2008, 12:41 PM
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#7 |
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its all about the +6 inches
Join Date: Jan 2001
Location: Hilliard Ohio
Posts: 2,690
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Re: Gear ratios?
the SPID is the option sheet in the glove box. Your 71 should have a 3.07
Seems odd to have a TH400 in there, that's why I was asking what was original. I don't think I've seen that in a 69 - 72 small block 1/2 ton. |
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01-22-2008, 12:53 PM
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#8 |
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67-72 Addicted
Join Date: Oct 2007
Location: Guelph Ontario
Posts: 256
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Re: Gear ratios?
Im about 99% sure its a TH400, i could be wrong though, but it was a z84 custom with all alot of upgrades. now say it where a TH350, would it still be 3.07 gears?
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01-22-2008, 01:17 PM
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#9 |
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its all about the +6 inches
Join Date: Jan 2001
Location: Hilliard Ohio
Posts: 2,690
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Re: Gear ratios?
yes it would be.
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01-23-2008, 12:22 PM
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#10 |
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67-72 Addicted
Join Date: Oct 2007
Location: Guelph Ontario
Posts: 256
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Re: Gear ratios?
Thanks, I'll check later this week if its a TH350 or 400, but on my SPID it dose have the M49 optional turbo-hydramatic, and also, if you say it has 3.07 gears it would be a GM 12 bolt, right? (i havent got to checking)
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01-23-2008, 12:42 PM
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#11 |
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its all about the +6 inches
Join Date: Jan 2001
Location: Hilliard Ohio
Posts: 2,690
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Re: Gear ratios?
I can't imagine it having a dana... I guess it could be possible, but probably a 12 bolt.
M49 could be either TH350 or TH400 in trucks. Last edited by Longhorn Man; 01-24-2008 at 10:03 AM. |
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