# cadillac rear axle gear ratios



## HARDLUCK88

i need to know 

1. what the gear ratio is for a 1988 cadillac vin y olds 307
2. what the gear ratio is for a 1984 cadillac with the 4100
3. what gear ratio is recommended for equal town / highway use with a th350


thanks!

i tried searching google, and its all bullshit...


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## HARDLUCK88

so far this is all i have come up with

Enter Potential Maximum Safe Engine RPM 3000
Enter Tire Size Diameter 24.33
Enter Desired Target MPH 70

Rear End Gear Ratio Needed 3.10

does that sound right?

randy's ring and pinion says these are the gear ratios for a th350

Turbo 350 2.52 1.52 1.00


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## budgetblueoval

most rear ends got a little metal tag on them that tell u what gear u have in it,, i bet u got one


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## HARDLUCK88

> _Originally posted by budgetblueoval_@Jul 14 2009, 10:28 AM~14467782
> *most rear ends got a little metal tag on them that tell u what gear u have in it,, i bet u got one
> *


my rear axle currently is from an 84 cadi that was fronted with a 4100, where do i gotta look?


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## biggeazy-e

> _Originally posted by HARDLUCK88_@Jul 15 2009, 12:10 PM~14481456
> *my rear axle currently is from an 84 cadi that was fronted with a 4100, where do i gotta look?
> *


pull the cover and check out the numbers stamped on the ring gear

you more than likely have around a 3.08 ring gear, since the 84 would've been an OD


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## HARDLUCK88

> _Originally posted by biggeazy-e_@Jul 15 2009, 05:15 PM~14484673
> *pull the cover and check out the numbers stamped on the ring gear
> 
> you more than likely have around a 3.08 ring gear, since the 84 would've been an OD
> *



shit, i really dont want to take the diff cover off. im afraid of what i might find.


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## biggeazy-e

> _Originally posted by HARDLUCK88_@Jul 16 2009, 12:30 AM~14489180
> *shit, i really dont want to take the diff cover off. im afraid of what i might find.
> *


gear oil, bolts and gears?


:0 :0 :0 :0 :0 :0 





















:biggrin:


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## jayoldschool

> _Originally posted by HARDLUCK88_@Jul 15 2009, 12:10 PM~14481456
> *my rear axle currently is from an 84 cadi that was fronted with a 4100, where do i gotta look?
> *


You look on the sticker on the underside of the trunk for the codes that start with "G". You will have GU6, and that means 3.42 gears. If you also have G80, you have posi (you probably don't). You also have the small 7.5" ring gear rear end that is much weaker than the earlier 77-81 Cadillac axle, or the 90-92 8.5" GM (Chev) axle.

If you are going with a TH350, you are losing the OD. Therefore, you should consider some higher (lower numerically) gears. Something in the 2.xx range. Swapping in a complete 77-81 rear end will get these gears, and will be much stronger. The 80-81 is the best choice because it uses a better driveshaft. You will need the drive shaft, too, and you will have to change the front yoke from the TH400 style to the TH350 style.

HTH
:biggrin:


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## HARDLUCK88

> _Originally posted by jayoldschool_@Jul 18 2009, 10:18 PM~14514477
> *You look on the sticker on the underside of the trunk for the codes that start with "G".  You will have GU6, and that means 3.42 gears.  If you also have G80, you have posi (you probably don't).  You also have the small 7.5" ring gear rear end that is much weaker than the earlier 77-81 Cadillac axle, or the 90-92 8.5" GM (Chev) axle.
> 
> If you are going with a TH350, you are losing the OD.  Therefore, you should consider some higher (lower numerically) gears.  Something in the 2.xx range.  Swapping in a complete 77-81 rear end will get these gears, and will be much stronger.  The 80-81 is the best choice because it uses a better driveshaft.  You will need the drive shaft, too, and you will have to change the front yoke from the TH400 style to the TH350 style.
> 
> HTH
> :biggrin:
> *



thanks alot man!!!!!!!!!!!!!


a few years ago i tried to get the rear disc brake axle off of a 78 sedan deville, but i couldnt get the driveshaft separated from the rear end 

i was told to try and get a 3.10 gear, are you talking high or low 2's here?

what gear comes in a 77-81 rear end?


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## budgetblueoval

> _Originally posted by jayoldschool_@Jul 18 2009, 08:18 PM~14514477
> *You look on the sticker on the underside of the trunk for the codes that start with "G".  You will have GU6, and that means 3.42 gears.  If you also have G80, you have posi (you probably don't).  You also have the small 7.5" ring gear rear end that is much weaker than the earlier 77-81 Cadillac axle, or the 90-92 8.5" GM (Chev) axle.
> 
> If you are going with a TH350, you are losing the OD.  Therefore, you should consider some higher (lower numerically) gears.  Something in the 2.xx range.  Swapping in a complete 77-81 rear end will get these gears, and will be much stronger.  The 80-81 is the best choice because it uses a better driveshaft.  You will need the drive shaft, too, and you will have to change the front yoke from the TH400 style to the TH350 style.
> 
> HTH
> :biggrin:
> *



did not know that


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## danp68

> _Originally posted by jayoldschool_@Jul 18 2009, 11:18 PM~14514477
> *You look on the sticker on the underside of the trunk for the codes that start with "G".  You will have GU6, and that means 3.42 gears.  If you also have G80, you have posi (you probably don't).  You also have the small 7.5" ring gear rear end that is much weaker than the earlier 77-81 Cadillac axle, or the 90-92 8.5" GM (Chev) axle.
> 
> If you are going with a TH350, you are losing the OD.  Therefore, you should consider some higher (lower numerically) gears.  Something in the 2.xx range.  Swapping in a complete 77-81 rear end will get these gears, and will be much stronger.  The 80-81 is the best choice because it uses a better driveshaft.  You will need the drive shaft, too, and you will have to change the front yoke from the TH400 style to the TH350 style.
> 
> HTH
> :biggrin:
> *


kinda depends on what your trying to achieve too-a 3.08 is a good all around gear for a th350(decend accel,decent highway gas milage)if you go with something like a 2.56 your gas milage will be great on the highway but you wont accelerate very quick or pull hills great without a running start-if you dont care about milage and dont run highway and wanna pull hills good and get a faster accel go with 3.73


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## danp68

:cheesy:


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## HARDLUCK88

> _Originally posted by danp68_@Jul 20 2009, 12:22 PM~14524926
> *kinda depends on what your trying to achieve too-a 3.08 is a good all around gear for a th350(decend accel,decent highway gas milage)if you go with something like a 2.56 your gas milage will be great on the highway but you wont accelerate very quick or pull hills great without a running start-if you dont care about milage and dont run highway and wanna pull hills good and get a faster accel go with 3.73
> *



its my daily driver, i do highway, and hills, here in connecticut. i dont mind doing 65 on the highway, lately i have been doing 50-55.

i just want to know what gear i am running now more than anything. i think it will help give me an idea on what some of my problems are...


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## Badass94Cad

> _Originally posted by biggeazy-e_@Jul 16 2009, 04:23 PM~14494868
> *gear oil, bolts and gears?
> :0  :0  :0  :0  :0  :0
> 
> :biggrin:
> *


Exactly what I was going to say! :cheesy:


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## jayoldschool

3.08s are a good choice. I have them in my 71 with a 350/350 combo. Decent acceleration, and not to many revs on the highway.


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## heavenlykid

are you trying to find ur current gear ratio or figure out what would work better. the easiest way this will get you close not exact is to mark the drive shaft and turn it 360 degrees all so at the same time draw a line on your tire and count how many times it rolls over. tada you have your rearend gear ratio like i said its a ball park not an exact. because hard to judge decimals by eye like .56 and .86 .83 on the mark.


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## HARDLUCK88

ok, so if i go make these marks and come back with results you can tell me approx what my gear ratio currently is? im also trying to get a lock on what speedo gear i need, im really tired of my shit being off by up to 30 mph...


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## jayoldschool

> *ok, so if i go make these marks and come back with results you can tell me approx what my gear ratio currently is?*


I already TOLD you how to check what your gear ratio is. And, I told you what the ratio actually is.


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## danp68

> _Originally posted by heavenlykid_@Jul 21 2009, 04:15 AM~14534965
> *are you trying to find ur current gear ratio or figure out what would work better. the easiest way this will get you close not exact is to mark the drive shaft and turn it 360 degrees all so at the same time draw a line on your tire and count how many times it rolls over. tada you have your rearend gear ratio like i said its a ball park not an exact. because hard to judge decimals by eye like .56 and .86 .83 on the mark.
> *


actually you count how many times your driveshaft has to turn to make your tire turn one complete rotation-say you turn your driveshaft a lil over 3.5 but not quite 4 complete turns to get your tire to do a complete turn,that would be most likely 3.73 rear gears-thats what the ratios mean,your pinion gear turns 3.73 times to get your ring gear to turn once or 3.73-1 ratio :yes: jack the car up and put it in nuetral,have a buddy tell you when the tire makes one complete turn while you spin the driveshaft and count how many times you spin it-not rocket science at all


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## HARDLUCK88

> _Originally posted by jayoldschool_@Jul 21 2009, 11:09 AM~14536822
> *I already TOLD you how to check what your gear ratio is.  And, I told you what the ratio actually is.
> *



it was a rhetorical question.


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## biggeazy-e

easiest way to tell (if the RPO sticker is missing)


remove the diff cover, check the numbers stamped on the ring gear


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