I vote for 4.10 gears. I've got 2 of them 2500 Suburbans with the 6.0 and 4.10s, a 2wd and a 4x4. LT265/75/R16.

I've towed about 5,000 lbs once and never felt bogged down. Actually, with 4.10 I feel are pretty peppy for being such a heavy truck. I believe they are a curb weight of 6,200-6,500 curb weight.

The 8.1 has more torque, so 4.10s should be a good combination. MPG for these trucks are somewhere in the 11-13 range empty, depending on engine, but within 1-2 of each other engine option. Less when towing.

Wasn't there a standalone controller for swapping a 4Lxxe to a 6Lxxe? The 90e being to more appropriate unit for higher GVWR of OP's vehicle.

The older GMT400 C3500 HD had the 8.1 and I believe those came with the NV4500 in the manual variant. Gear ratios for those heavy beasts were like high 4 (4.83) and 5.13s? Something like that. You get the idea.

 

only the 2002 c3500hd (supposedly)... all other GMT400s 2001 & earlier had the 454 as the big block.
but yeah, that body style used the NV4500 for the heavier trucks with the 1/2tons typically running the NV3500 (though some years let you order the 4500 in the 1/2 ton, if i remember right). the 6 speed was only GMT800 8.1 or diesel.

 

I thought it was 01-02 that had the 8.1 on those trucks, but year, very limited.

I was thinking OP could swap to the 6L90e (with that aftermarket controller), which would be similar to what the GMT900 2500 Suburbans and Express Vans had. 4.03 first gear and split overdrive gears? I can't comment on the reliability of one vs the other unit though.

 

I might be interested in the 6l90 if it was an easy swap. However, I prefer manual transmissions. I really don't like automatics. So, from my research the 6l90 requires an expensive controller. Drive shaft and cross member mods. I'm not sure if that controller even works with the 8.1. So by the time I figure all this out, I'd rather just go with the NV4500. Unless there is a Richmond 6 speed or something out there capable of 10,000 pound towing with similar ratios to that 6l90.
As it stands I've decided to go NV4500 and drop the rear gears to 3.21s. With electric fans and the intake manifold mod I hope for 15 mpg on the highway.

 

 

It would work the 8.1

 

well f--- me!!

after a little more digging, i actually found a couple instances of some 2001 c3500hd trucks with the L18 (3GBKC34G81M110325 is one of them). additionally, the other c3500hd confirmed GM kept the nv4500 behind it as a factory 3-pedal option for those who didn't want the 4L85, just like they did with the 454, and can be seen here: https://www.thetruckstop.us/forum/th...y.46002/page-2

still, the only zf s6-650s were in the GMT800 big blocks & diesels