Trying to figure out if I will need to regear my rear end

[D-n-Tyke]

I am trying to figure out (and then possibly budget) if I will need to regear my rear end. Going to be running a 6.0/4L80E. Current gears in the rear end are 3.08. Anyone running this set up and having it work well for them. I would like to get half way good gas mileage, well as best as I can thru gearing, not running high RPM on the freeway, but also don't want it to be a slug off the line. What do you guys suggest, run what I have, or regear it?

4L80E Ratios
1st 2.482
2nd 1.482
3rd 1.000
4th 0.750

R&P 3.08

[72SB]

FWIW I had 3:08's with that ratio of trans in a 4200lb car with the 6.0 (383). If was fine. I felt the best combo was 3:42 though.

[kwmech]

Tire size plays a huge part---for example I am running a 285/75r-16 which is about 33 in diameter and running a 4.56 ratio with a 4L80e

[Jason Banks]

That'll be the gas mileage king with 3:08. I have the same combo 6.0 and 4L80e with 3:73 rear gears. It is totally comfortable at highway speeds(relatively low rpm) and will go 100mph effortlessly.

I'd say get it running the the 3:08 and see how you like it. If you want more performance change it then.

I only ever had one truck with 3:08 rear end was at 402 big block with a 3 on the tree. It was a great highway truck, even without overdrive.

[Accelo]

2.482 * 3.08= 7.64 final drive ratio.

The 4L80 has the same 1st gear ratio as the THM400.
As the GM transmissions evolved first gear got lower, over time.
Will this work? Sure it will. But don't expect tire burning acceleration.
This will be just like the factory gearing if the transmission was a THM400 & 3.08.
Since you would have to run a transmission controller of some type as all 4L80 require a computer, to run, you might consider a 6L80.
First gear in the 6L80 is a 4.03 ratio. That times the final drive (3.08) gives you 12.41 ratio.
Second gear ratio, on the 6L80 is 2.36. It's so close to first in the 4L80 you can consider them the same.


10 to 1 ratio is considered an optional acceleration ratio. I prefer closer to 8 or 9 to 1 ratio.

To recap;
The 4L80 1st gear is a little high and the 6L80 is a bit low when using the 3.08 gears.
The 6L80 also has over-driven 5th and 6th gears so it drops the RPM at speed a lot.
The 6L80 is like a THM400 or a 4L80 with a much lower 1st gear and two overdrive ratios.

Your choice, guess it would depend on if acceleration or mileage was was the most important criteria, and of course cost.
Cheers

[D-n-Tyke]

Originally Posted by kwmech

Tire size plays a huge part---for example I am running a 285/75r-16 which is about 33 in diameter and running a 4.56 ratio with a 4L80e

Crap. Thought I put that in my original post. Tire are 275/40r20 (28.7in)

[D-n-Tyke]

Originally Posted by Jason Banks

That'll be the gas mileage king with 3:08. I have the same combo 6.0 and 4L80e with 3:73 rear gears. It is totally comfortable at highway speeds(relatively low rpm) and will go 100mph effortlessly.

I'd say get it running the the 3:08 and see how you like it. If you want more performance change it then.

I only ever had one truck with 3:08 rear end was at 402 big block with a 3 on the tree. It was a great highway truck, even without overdrive.

That is ultimately my plan. Get it running with what I have. But also want to keep in mind budgeting for gearing if I am going to have to change them on the future.

[D-n-Tyke]

Originally Posted by Accelo

2.482 * 3.08= 7.64 final drive ratio.

The 4L80 has the same 1st gear ratio as the THM400.
As the GM transmissions evolved first gear got lower, over time.
Will this work? Sure it will. But don't expect tire burning acceleration.
This will be just like the factory gearing if the transmission was a THM400 & 3.08.
Since you would have to run a transmission controller of some type as all 4L80 require a computer, to run, you might consider a 6L80.
First gear in the 6L80 is a 4.03 ratio. That times the final drive (3.08) gives you 12.41 ratio.
Second gear ratio, on the 6L80 is 2.36. It's so close to first in the 4L80 you can consider them the same.


10 to 1 ratio is considered an optional acceleration ratio. I prefer closer to 8 or 9 to 1 ratio.

To recap;
The 4L80 1st gear is a little high and the 6L80 is a bit low when using the 3.08 gears.
The 6L80 also has over-driven 5th and 6th gears so it drops the RPM at speed a lot.
The 6L80 is like a THM400 or a 4L80 with a much lower 1st gear and two overdrive ratios.

Your choice, guess it would depend on if acceleration or mileage was was the most important criteria, and of course cost.
Cheers

A lot of GREAT info there. Thanks for explaining all that. I wish I could afford going to the 6L80 but don't think thats in the buget right now. Think I will try out the 3.08 and see how bad the acceleration is and then go from there. Thanks again for taking the time to write out all that info. I am sure I will be refering back to it down the road.

[jocko]

Below are your cruising RPMs at 55,65,75 mph with a 28.7" tire, 3.08s, and a 4L80. GM normally put something in the neighborhood of 3.73 with newer O/D transmission. Your combo is doable, but RPM is pretty low at 55 mph cruising.

[KQQL IT]

If its making any decent power 3.08 will be fine with that tire.
And will accelerate just fine.

Unless you're planning on drag racing, i wouldn't change anything.

[D-n-Tyke]

Thanks for the input everyone.

Small correction. Tire size is actually 295/40R20 which is (29.3"). Don't think that will make a massive difference but <shrug>

[tim_mc]

I'm running a 700R4 and had my diff rebuilt from 3.08s to 3.42 due to some lugging at lower speeds and a few too many downshifts on the highway. In all honesty, I was looking for an excuse to put in a posi rear end. I mostly scoot around locally and wanted better acceleration but didn't want to pull too many RPMs on the interstate. With 3.42s I'm cruising 70 MPH on the highway at 2000 RPM with a tire size of 235/55R17 (~27"). Like Jason and a few others suggested, I would try the 3.08s first as it may be perfect for your driving needs.

[Myself]

I prefer a bit more rpm myself. I live in the hills and there's a lot of 55mph driving around here. When you catch those hills you have to downshift or anticipate the hill and smash the pedaal more so it downshifts itself. Otherwise it's lugging.
So, for hilly areas like mine that's too high geared and the 3:42 works much better.

[Wrenchbender Ret]

I think you will love the 3.08 with o-drive. I ran a Stepper for a long time like that. If are pulling a load or in hilly country you can pull it down a notch & drive in straight through.
I had a long hill on the interstate near home. It would pull it in o-drive w/ac on without downshifting. It was a pleasure to drive.

George