I own an '81 GMC Jimmy, which originally came with a 305 but had an engine swap to the 400SB with dual exhaust, done by the vehicles PO. Its also got a 6" suspension lift, a 3" body lift and 40" tires, again from the PO.
My problem is that the trucks got no guts, and really isn't quick - up a hill it loses speed and I can't help thinking it shouldn't be like this, even for an old 4 speed vehicle. I was originally thinking that I would need to squeeze some more HP out of the engine but upon further reading I discovered that it could be down to the axle gearing, could this be correct?
I read something along the lines of: "When lifting a vehicle and adding oversize tires, owners neglect to adjust the axle gearing."
If this is right, does anyone know what kind of gearing I should go for in the axles?

Thanks in Advance.
Jamie

 

Start with determining what you already have. You can pop a diff cover and either count the teeth or find the stampings on the ring gear. Or you could mark the driveshaft and count the number of revolutions of the driveshaft it takes to spin both wheels once.

 

It's roughly 3.25 spins of the driveshaft to rotate the wheel one complete turn...

 

Well your choices are 3.08 (likely) or 3.42 (also likely). To which side of 3.25 is it? High or low?

Either way, your gearing is way too high to spin those 40's effectively. You should be looking to go at least to 4.10's. The problem is that will require changing the differential carrier since the higher ratio carrier (lower numerically) has a greater distance from the pinion to support the higher number of pinion teeth.

Open carriers are not all that expensive. I need to pick up a different carrier for my Dana 44 to install the 4.10 gears I have when I put my 14bff under my K5.

 

If it's a 4X4 an you change the gears, you must match the front and rear to the same ratio.

 

So 4.10 is the ring and pinion size i need? Can you please explain how it works, I mean the higher this number, does that affect accelleration or top speed? What happens if I go above 4.10 to say 4.56 or 4.88?

Also I found these:

http://www.4wheelparts.com/Drivetrai...=G%2fGGM12-410

http://www.4wheelparts.com/Drivetrai...G%2fGGM12-410X


*Notice the first says its for use with 3.73 and up carriers and the second says its a "thick 4.10" for use with 3.42 and down carriers. If I brought a pair of the second set will I still need new carriers?

 

If you want to go over 4.10's, you are going to have to get new carriers. They are not too much $$$.

If you were sticking with just 4.10's, then the second link would work, but would not be as strong as the proper carrier with the regular 4.10 gears.

I would recommend 4.56's for those 40's, but you really are going to be pushing the axles hard with 40's.

 

Hey Kyle, Thanks for getting back to me on this.
Doesn't changing the gearing lower mean that the engine will be revving higher thus earlier gear changes are needed and the cruising revs are higher at highway speeds?
Also this means increased engine wear and worse fuel economy right?
What am I getting thats a positive here? Haha.
Will the truck not feel so sluggish? Will it not lose so much speed on steep inclines?

Anyhow, I found this..

http://www.4wheelparts.com/Drivetrai...G%2fGGM12-456X

A 4.56 for 3.42 and down carriers.. What did you mean about this putting alot of strain on the carrier?
Where can I find new carriers and how will I know that the new one won't suffer alot of strain?

 

Being extra thick pushes the carrier further from the pinion which is not good.

Considering that your truck came standard with ~29" tall tires, you are changing the final drive ratio by 38% with tires alone. Going to 4.10's will bring that down to 18%. Going to 4.56's will bring that down to 5%.

To put that into perspective in terms of engine RPM at 60MPH in direct drive (1:1):

• 29" tires w/ 3.42 gears - ~2400RPM
• 40" tires w/ 3.42 gears - ~1720RPM
• 40" tires w/ 4.10 gears - ~2100RPM
• 40" tires w/ 4.56 gears - ~2300RPM

 

Ahh perfect.
Thanks for that Kyle, thats awesome. Now things are starting to make sense.
Sorry i'm a noob when it comes to this but thats definitely cleared things up.
So what are the carriers? Is that the complete axle, axle shafts and differential?
How do I know they will be better than what I have? In other words, what am i looking for here?