Originally Posted by _zebra

i'm in the same boat & would've bought a Escalade or Denali to stick-swap if we hadn't found our H3. they've all got the same cut-out spots for the clutch master & shifter as the pick-ups & can easily swap interior pieces to look like a factory job.

first-off, i'll clear up some misconceptions before giving my opinions on the matter:

• the ZF S6-650 and NV5600 are completely different transmissions; the former was made by ZF and was a factory offering behind the 8.1 & Duramax pickups, while the latter was made by New Venture Gear and never offered in GM trucks (because they chose the ZF over it).
• you mention the NV4500 basically being a 4spd + a granny (technically more like a 3spd + granny + overdrive [since 4sps usually ended in 1:1])... but this ZF is the same way. "first" is a deep granny (because it was designed for diesels using a high 3.42 axle ratio), so it's normally driven as 2-6 (with 5th being your direct gear, i.e., single overdrive).
• all the truck transmissions of this era have almost the same overdrive ratio (even ones with a couple application-specific gear set options throughout their runs).
  • 4L60/65 = .70
  • 4L80/85 = .75
  • Allison 1000 = .71 (2006+ added .61 double OD)
  • NV3500 = .73
  • NV4500 = .73
  • ZF S6-650 = .72
  • NV5600 = .73
• using a Corvette ZF S6-40... just NO! it was designed for a 3400lb car & would burn right up in a 1/2ton Suburban by itself, let alone trying to tow with a 2500 (just like every guy who tries a T56 in that kind of application).

as for my own thoughts: had the 2500 SUVs been offered with an Allison instead of the 4L85, it would've already had the 2" body lift (only the Allison & ZF physically needed it)... so if you were to do a lift, the factory bumpers, brackets, & filler plates to make it less obvious would be a relatively-easy swap from a pickup.

the NV3500 from your 4.8 truck could take the 8.1's power... it'd just quickly die under the loads discussed here (i added my thoughts/experiences to your other thread about that truck). to quickly address the NV5600 idea: never heard of anybody trying it, but you might could swap a NV4500 bellousing over & make it work, but no clue on comparing the hydraulics, mounting modifications, or driveshaft length needed to make it happen = proceed at your own risk after a ton of thorough study.

given the data you've provided, my recommendation would be to still do a 4.11 swap (or at least leave the 3.73s). the few hundred RPM difference in OD will be offset by better mechanical advantage (less load on the engine), so the highway mileage shouldn't change much while increasing performance (especially with the 32" tires and driving around town in the lower gears). for a towing rig, you don't want to gear the axle for a DD-able granny - that just leads to lugging around in OD & straining everything more in a 3-3.5 ton truck (now, if this was an RCSB will all that torque, i'd sing a different tune).

that said, since what you described towing above (~14k gross) wouldn't require the 26k of the ZF, i'd recommend the NV4500 (14.5k GVWR). the Suburbans & Avalanches never had the Allison or ZF, so they ran the same body height as the 1500s over the frame... which is as simple as using the GMT400 transmission case lid instead of the 01-06 version that accommodated the 2500's body lift. just make sure you also get the correct shifter for the lid (personal suggestion is forking over the cash for Core's Hurst remake - totally worth it!).

if you're worried about heat when loaded up, the NV4500 can use the same trans-cooling PTO cover extensions as the ZF & 5600, too.

Originally Posted by Topfuel67

Thank you for the reply here too. I have researched this and have come up with the same. I was hoping there's some magic 6 speed out there I haven't come across yet. I do prefer the ratios in my NV3500, I just doubt it could handle my car hauler loaded up. If I was only going to tow my < 5000 lb. family RV I may have considered it. What it comes down to is I'd love to drive the suburban more than just towing. The way it is, she gets 11 MPG towing or not. Towing light or heavy, she doesn't care. That 8.1 is a beast. I've seen that I can drop the rear axle to 3.42 or 3.21, but wouldn't touch that unless I had the granny gear. I also figure with a manual I can stay in 1:1 easily if needed while towing. I read a write up a while ago where a guy swapped a zf6 from the diesel truck into his 8.1l avalanche, but that was a 4WD. I think those come with the body lift needed.
If my only option is to body lift, swap rear gears and do manual trans swap, I'm not sure it is worth it. It really is hardly worth it to go through all that is required for the manual swap alone. By the time I do pedals, drill for the slave, modify for the console shifter it is getting pretty involved.

Originally Posted by dantheman1540

@zebra nailed it on this one. I’d NV4500 it and not have to touch gears, probably the cheapest option as well.

Originally Posted by Topfuel67

I like your Tahoe in your sig! How does the nv4500 perform in your Tahoe? MPG? Do you use first gear with the 3.42 rear or just 2nd-5th? I was considering the NV4500. I'm guessing if it was used behind a 454 in the 90s that bellhousing would fit my 8.1.

Originally Posted by dantheman1540

not the case with my RCSB.

Originally Posted by dantheman1540

I like it a lot and feel that the “useless” 1st gear is actually awesome to have. So around town 99% of the time I don’t use first gear, the exceptions are, when towing, if I want to leave from a light super hard, or if I’m in stop and go traffic and just want to creep. I can use second gear when towing, but I feel less clutch slipping is better for clutch longevity. I don’t remember if your truck is 4x4 or not but having the low first gear is really awesome for off-roading.

MPG with a smaller cam and head combo plus smaller tires was about 15city/19highway

Current mpg with 33s and bigger cam and ported 243 heads is 13city/16highway. I could probably squeak out better highway mpg, but I don’t do long highway trips often, and when I do I’m pushing 85mph.


As far as the clutch goes I’ve had fantastic luck with LUK (pretty sure that’s the name). It’s basically an OEM style replacement nothing fancy at all. It’s quiet, very forgiving, no chattering, great pedal feel, and I beat the crap out of it. Including the miles from the previous owner I believe it has around 60-70k miles on it and I’m not expecting replacement anytime soon. Seriously I beat the crap out of it, lots of clutch dumps, burnouts, slides, trailer launches……

The things I don’t like about it are:

-The nv4500 in my Tahoe is noisy compared to the other two nv4500s I’m familiar with. I’ll sum that up to a poor rebuild tho.

-It’s not a T56 or any other car transmission. This is something best related to a tractor IMO. That’s not to say you can’t bang gears because I rev mine to 6,900 and can (if I don’t suck that day) bang shift it really well. 3rd is the only gear that occasionally will grind a tad but it seems to only be the nv4500 in my Tahoe and not the case with my RCSB or my buddies NV swapped Denali. Again I’ll sum that up too not the best rebuild.

-Heavy, this **** heavy as hell if you have to pull it don’t expect to bench press it into place it will crush you.


Lastly I’ll leave you with this, with 3.42 gears and an exactly 33” tall tire here is how long I can run the gears.

1st: 30mph
2nd: 58mph
3rd: 85ish
4th: 100+
5th: infinity and beyond


**Edit**

Also no body lift required for any of the three trucks in know that have them.

Originally Posted by Topfuel67

I hate automatics. I really hate the 4L80e. It has a 2.48 first gear ratio. That is ancient technology. It is ridiculous that GM used this all the way into the 2000s. The 60s Muncie 4 speeds even used a 2.5 first gear. My 01 8.1L suburban tows great. But with the power of the 8.1L is a dog off the line. I blame the 2.48 first gear ratio. I have the 3.73 rear end. I have researched swapping a zf6 from the same year trucks, but think they went too far the opposite direction on that trans with a 5.79 first gear ratio and still only .72 for the o/d. I would prefer having a .6 or lower o/d to save fuel on the highway. Also I have read that the zf6 is so big it requires 2-3" of body lift on the subs. My sub is 2wd and I'd rather not lift it. I do tow with my sub. I have a 24' car hauler and a 5000 lb rv trailer. So I am wondering if any of the other transmissions out there are capable of handling this power. I've seen the tremec and other 6 speeds advertise that they can handle 700lb+ of torque which is far above the 8.1, but that's not the towing load. I am in California so I don't have to worry about smog. I don't care if I have to disconnect the trans controller. I have looked at the 6L80 and that seems like I would need to change the engine to a 07+ 6.0l which I'm not willing to do. Plus, I still hate automatics. Has anyone used any other 6 speeds to tow and how much weight?

Originally Posted by Fast355

Is your 8.1L tuned?

I currently tow with a small block that makes ~500 ft/lbs, a 4L85E and 3.73 gears. Travel trailer is 6,000 lbs and the tow vehicle is usually 6,200-6,500 lbs. I have a high str 2,800 rpm stock diameter converter with a heavy duty single disc and I lock it up in 2nd gear shortly after the 1-2 shift in tow/haul. The 85E has been dead reliable for me, same cannot be said for the numerous 6spd automatics.

A good tune can really wake the 8.1L up and make the 4L85E feel and operate alot more manual trans like. Shift schedule and TCC lockup schedule make a huge difference. A good converter can also really take the slush box out of the 85E as well. A manual trans even with a lower gear ratio offers less breakaway torque than an 85E when the converter is multiplying torque. The close ratios of the 80E trans also worj very well towing. When I had a lesser 350 I ran a 5.13 gear and it was night and day difference compared to a 3.73. Having another full gear to pull in made the low-speed acceleration much stronger and it made for less engine rpm on long grades because I never found myself in 2nd gear at 60+ mph. Even 4.10s gave the 8.1L/85E another 2,000 lbs of towing capacity.