4l80e /NP205 with 35” or 36” tires?

[SONOSADSACK]

Anyone running a 4l80e with a 205 behind it with 35” or 36” tires? What gear are you running in your diffs and what kind of rpm are you seeing at highway speeds?

[1971gmcshortbed]

I am about to do one for my brother, have you done yours? Did you/are you using Advance adapters kit?

[Thealien]

I am interested what you guys will be doing with your shifter. Also are you just moving the transfer case back or the engine forward also.

[SONOSADSACK]

I not 100% sure about anything yet, just kind of feeling my way through this as all I am finding for 6.0 swaps applies to 2x’s. I have my engine, trans and t-case and also know I want to run 35” – 36” tires. I thought I should determine tire size first to help determine what gear I’ll be using. Just got off the phone with my tranny guy who is suggesting I use a 4:88 gear. I wasn’t thinking anything near that steep.??

Thealien, so far I’m planning on going with the stock t-case mounting location. You?

[SONOSADSACK]

Short bed, I planned on ordering AA P/N 50-0410 this week but my tranny guy has a friend that he believes did something different and told me to hold off on the order until he had a chance to get a hold of him. If I do go with AA I need to find out if they have a cheaper kit if you use a 32 spline 205 rather than the 27 spline 205.

[1971gmcshortbed]

Wow, this could be a heck of a thread. Thealien, are you talking about the trans or the transfer case shifter? According to Advance Adapters it moves t case back 3.5 inches. Sonosadsack- have you read the instructions to AA P/N 50-0410? You have to use a full spline 4L80 output shaft that is shortened in your 4L80. They dont supply it in the kit and it's discontinued through the dealer. The dealer found it through vintage parts but I had them email me a pic and it wasn't full spline. Which means it wont work. It's a lot of work and changing parts etc.
I emailed AA a pic of my shaft before I cut it to make sure it is correct. I just did this today, I hope to hear back from them soon. Also you have to replace the front bearing in the 205 which requires some machine work I believe. What year is your bb/4L80 (or 6.0)?

[extd56]

I am running a 4L80 in my my crew cab truck but using the 241 transfer case. I have a big block for power and running 3.73 gears with 35" tires. My RPM's are about 2000 at 70MPH. It seems to pull the truck without a problem.

[Critter]

I'm in a similar boat. I have already bought my 6.0 and I plan to go with a 4L65e. I originally was thinking about a 4L80e but after giving it some thought I realized that the drivetrain would be too long for my taste. If you keep the Tcase in its original location, the engine will end up being 6 inchs (give or take) farther forward than stock. If you keep the engine in the factory location, the Tcase shifter ends up coming through the floorboards between the seats. I guess you could split the difference and make it work???

Anyway, I used this calculator and my knowledge of what most of these 6.0s ran on the streets from the factory to determine I want either a 4.56 (2200 rpms @ 70mph) or possibly 4.11s (2000 rpms @ 70mph) with my 35 inch tires. Keep in mind I will basically just be on the highway and trails, no rocks or deep mud. I guess it depends on what kind of driving you plan to do.

[vtblazer]

Best thing to do IMO, is to run the numbers.

Figure out what you have and decide what you want.

http://www.grimmjeeper.com/gears.html

[SONOSADSACK]

Today I was able to get a hold of a very helpful tech at AA. According to him, if I have a 32 spline NP205 with 3 5/8” (not 2”) of input shaft sticking out of the front and if I have a later model four wheel drive 4l80e then all I will need is their replacement adapter. I would also have to add a reluctor ring to the output shaft in the 4l80e but this will be necessary regardless. What this also means is that I wouldn’t have to machine the t-case to replace the input shaft or replace the output shaft in the trans or spend $621.00 on the complete kit. I do have a 32 spline t-case but I’m not sure how long the input shaft is. I should be able to get out tomorrow night and pull the adapter off of it to measure. My fingers are crossed as this sounds too easy.

VT,
Thanks for the link, super easy end very helpful. Now if only deciding what I want were as easy!!

[Firebirdjones]

That's the same swap I am collecting parts for. LQ4/4L80E.

As far as gears you will have to run some numbers and see where you want to be.

I currently have 3.08 gears and 33 inch tall tires. The tires are staying. I'm moving the gears up to 4.10 though. The overdrive isn't as bad in the 4L80E,,,,it's only .75

I've done these swaps before with 4L60E's and their .70 overdrive. I went with 4.10 gears also along with 32 inch tires in that case.

I prefer to keep rpms around 2200 to 2500 rpm cruising highway with overdrive trannys. Especially since there are alot of hills to climb around here. I can't stand the silly thing kicking in and out of overdrive everytime I have a grade to climb. So I tend to gear it a little steaper.

Currently with 3.08's and 33 inch tires with the 350 turbo tranny,,,,,It cruises at 70 mph at a nice 2400 rpms (no overdrive) and it's just about right. Any lower in rpm and I would be looking for passing gear to climb some of these hills.
If the truck wasn't so heavy and shaped like a brick it wouldn't be as bad,,,and I could lower the cruise rpm a bit.

With all that said,,,depending on how you drive and what hills you have to deal with (what you are willing to put up with) I'd say your tranny guy is pretty close with your intended 35-36" tall tires.
With that tall of a tire I'd personally pass right over a 4.10 gear (as I've run that with 32" and it's was great) And I'd shoot for 4.56 or somwhere in that neighborhood. Even the .75 overdrive knocks that down to a measly 3.42,,,couple that with 36 inch tall tires and the thing is going to cruise nicely down the highway at only 2,234 rpms. With a 35 inch tall tire that only makes the rpm climb to a comfy 2,300 rpms.

Dropping to a 4.10 with a 36 inch tall tire would drop the rpms to 2,000 at 70 mph. A bit low for my taste, especially in a heavy boxy vehicle.

[Raindoc]

Sonosadsac:
I have a K5 4L80E/208 with 33's and 4:10. I calculated it and determined that I would be turning 1900 rpm at 65 mph. I would think you would be down to about 1700 1800 rpms with 35" or 36" tires. I think that is too low.
As far as positioning, I left the engine in the back location and moved the T-case crossmember back. The 208 hangs down a little more than the 205 which meant I had to notch the cross member a little. I have a 2" lift which provided a little challenge with driveshaft angle (with the longer trans). I went with a double cardian style u-joint at the rear of the t-case and rotated the rear pinion up to within 2 degrees of what the driveshaft is. This worked out real nice. I have not run it yet but it should be very smooth.
I have not solved the shifter issue yet but I am open to suggestions.