No Uumph

[1978K10]

Ok,
bought this 78 Chevy in my profile 3 years ago. When I bought it, the gears from the rear end were laying on the ground-the previous owner had shot the spider gears out the back of the pumpkin. I think they were 3.73.

Had it towed to a repair shop and asked them to put another rear end in it. They did- a used one from a junkyard. I vaguely remember them saying they put a either a 3.56 or a 4.56 in it. I shrugged not knowing too much about 4x4 when i bought it and figured these guys had a clue as to what they were doing- The thing ran and drove again just fine. Back even then it never really had a whole lot of ummph. The engine is a SBC 350, rebuilt 40K ago- rvcam, 30 over bore- typical rebuild
Since then I have replaced-

• 3" Hooker Headers
• Straight pipes Custom 3" exhaust- Flowmaster offroad 40's
• Edlebrock 650
• 454 Radiator
• plugs, wires
• belts
• High Amp Alternator
• 850 CCA battery
• gaskets all around
• fluids all around
• Removed the AC
• Clutch Fan and Shroud
• 4" lift-replaced all shocks
• 35's on it
• Put alot of blood and heart into it
• body work etc.

I can't make the tires chirp for the life of me- there is no ummph, even when i punch it i would think the whole truck should jump and take off like a bat outta hell... although it runs fantastic, sounds beautiful and has plenty of power- it feels like it's reserving. This thing was great in the blizzards of 07 in Denver. Pulled out trucks bigger than mine, no problem.

Here's what i am now trying to understand. I have looked at the front gears and believe it's a Dana 44 and I believe with 3.73 gears in it. Did that shop i originally took it to put a different gear ratio in the back end and therefore my acceleration is dragging? Can this even be possible, can you put any gear ratio you want into the back end of it? Don't the gear ratio's HAVE to match? Do people do this? and I am looking in the wrong direction?

Please help, I'm so lost and starting to get aggravated at that shop and starting to think that shop phucked me and now I have to replace either the front end to match the back or replace the back end with something that matches the front...

 

[4X4HIGH]

First off, gears must match front and back otherwise something's going to break.

Also, you need to figure out exactly what gear ratio you have (preferably in the rear) since this is the axle that moves you around in 2wd. Now we need to know what size tires you're running on it so we can tell if the tire to gear ratio is giving you problems. Once we know this info we can better help you with your problem.

 

[1978K10]

ok,

I have 35's on it now
I'll crawl under it tonight and see if i can find any markings on the diff, otherwise after work tomorrow a friend of mine will help me count the rotations on the driveshaft trick. Thanks for the assistance.!!

I'll have to spin the tires tomorrow, there are multiple numbers and such stamped on both front and back. I googled for answers but turned up nothing.

What i found tonight-

Back is a 12 Bolt- GM 1 | Either A1810 or A13 or A18- not sure if the 1 and 0 were numbers or just part of the stamp- On the bottom passenger side ear of the front of the pumpkin there was a 9969

Front is a 10 bolt-only numbers stamped on here- one big 2, under that- 37838

 

[throttleissues]

The only way to know for sure the exact gear ratios is to pull the covers off and look at the numbers stamped in the ring gear. i.e. 41-10 for a 4.10:1. Those numbers tell you the gear tooth counts for each gear, then you divide to find the ratio.

 

[imiceman44]

The only way to know for sure the exact gear ratios is to pull the covers off and look at the numbers stamped in the ring gear. i.e. 41-10 for a 4.10:1. Those numbers tell you the gear tooth counts for each gear, then you divide to find the ratio.

Or he could do the :
....I'll crawl under it tonight and see if i can find any markings on the diff, otherwise after work tomorrow a friend of mine will help me count the rotations on the driveshaft trick. Thanks for the assistance.!!

 

[throttleissues]

Or he could do the :
....I'll crawl under it tonight and see if i can find any markings on the diff, otherwise after work tomorrow a friend of mine will help me count the rotations on the driveshaft trick. Thanks for the assistance.!!

I thought that was the "I'm gonna drive over a friend while he counts d-shaft revolutions" trick.

 

[1978K10]

So after the big run over my friend test

We found the diff turned a little over 3.25 times, after looking at the possibles in a 12 bolt, correct me if i am wrong...

Part# Ratio Teeth
GM12-308

3.08
40-13
​

GM12-342

3.42
41-12
​

GM12-373

3.73
41-11
​

GM12-373X*

3.73
41-11
​

GM12-411

4.11
39-9
​

GM12-411X*

4.11
37-9
​

GM12-456

4.56
41-9
​

I am guessing a 3.42?

 

[imiceman44]

So after the big run over my friend test

We found the diff turned a little over 3.25 times, after looking at the possibles in a 12 bolt, correct me if i am wrong...

Part# Ratio Teeth

GM12-308

3.08

40-13​

GM12-342

3.42

41-12​

GM12-373

3.73

41-11​

GM12-373X*

3.73

41-11​

GM12-411

4.11

39-9​

GM12-411X*

4.11

37-9​

GM12-456

4.56

41-9​

I am guessing a 3.42?

I am going on limb here but the 12 bolt was mostly common with 3.08 and 3.73, the 3.42 got offered on the 80's trucks so I would tend to guess 3.08
The question is what did the front show?
You need to know if they match.
The 3.08 would exp-lain the gutless truck, there is no way you can chirp the tire with a 3.08 axle.

 

[1978K10]

All highway gears eh?

 

[4X4HIGH]

Stop guessing, drop the diff cover and get the info off the ring gear or count the teeth and do the math. You could have already figured out what gears are in it in about 5 minutes if you had just pulled the diff cover. Probably not a bad thing to do anyways since you can get a look at what things look like inside and then when done fill with fresh gear oil.