Eaton Rear End Gear Ratios

[Beat68CST]

I have 4.56's right now.

Is lowest I can go 4.11's?

Some 3.90's would be awesome.

[Longhorn Man]

the lowest you can go is a 5.13.
the highest you'll find is 4.10
there's some 3.90 gears out there, (aftermarket) but they are very far and few between.
Doing the junkyard crawl, look for small block or big block trucks with an automatic, then double check to make sure it is a 4.10 and not optioned otherwise. (4.10 was standard in V8/auto) Don't forget to look at the one ton trucks too.

[Beat68CST]

I knew you'd be the one with a definite answer for me!


Who manufactured the 3.90's and do you have any idear of where I might be able to find a set?

[Fitz]

Originally Posted by Beat68CST

I have 4.56's right now.

Is lowest I can go 4.11's?

Some 3.90's would be awesome.

Depends on the rear you have. I assume your diff is an 8.875 12 bolt truck. Large carrier goes from 3.73 to 5.12 Small from 2.76 to 3.42

[Stocker]

Originally Posted by Fitz

Depends on the rear you have. I assume your diff is an 8.875 12 bolt truck. Large carrier goes from 3.73 to 5.12 Small from 2.76 to 3.42

Per the thread title, his diff is an Eaton (GM Corporate). Andy nailed it.

[Shaky]

Originally Posted by Beat68CST

Who manufactured the 3.90's and do you have any idear of where I might be able to find a set?

I think Yukon manufactured them. Maybe if we all keep calling them they'll do another run of 3.90s.

[Beat68CST]

Originally Posted by Shaky

I think Yukon manufactured them. Maybe if we all keep calling them they'll do another run of 3.90s.

Yeah, that'd be awesome, I'd actually go for something a bit lower too, even these 3.90's would be spinnin a Cummins pretty good.

[Longhorn Man]

You could probably locate a 14 bolt with 3.73 or even better. Shouldn't be too hard to swap in there if you are able to install a cummins

[6772king]

i put a dana 70 with 307s in mine and it was perfect,but i didnt haul real heavy loads,, I agree that even 390s with the cummins would be to much.

[Longhorn Man]

I wouldn't put a 67 - 72 chevy C/20 dana in there though. The axleshafts shatter way too easy. (not the dana70 mentioned above)

[Beat68CST]

Well, even though I'm an 18 year old kid, I'm not much of one for hauling @$$ everywhere.

My dad and I drag race for a reason, lmao. We don't need radical street cars.



If I can cruise 65mph at 2k RPM I'll be more than happy. That's what I'm shootin for.

[Longhorn Man]

you don't need to be hauling ass to shatter an axle shaft. A heavy load, or just romping the throttle. Large tires, steep hills, torquey motors... each can do it, a combination of them is asking for trouble.

[capev86]

my 72 Burb C20 turns 3,000rpm at 60mph with stock tires. that is with the 4.10 ratio swap which is 90% of the original 4.57. beauty of the Corp axle is the removable 3rd member (like a Ford 9 inch). only using the 3.90 aftermarket gear would require you to take the diff apart rather than just swap for the unit with the gears you want. the 3.90 is only 5% softer than the 4.10 and probably about equal to a 1/2 ton (w/ the stock shorter tires) with 3.73 gears.

i heard a 6.13 ratio was available also (probably only in the HO72 in the 1 ton trucks).

and the 4.56 and 4.11 are ratios found in 1/2 ton trucks....different axle

[DeadheadNM]

I didn't know that about the C20 Dana 60's. I just got one from a 70 camper special and new spring perches to be installed in my K20 in place of the Eaton. Bummer.