More gears are better, right?

This is more of an I wonder than an I plan. 

My HT3 has the 3.7 I5 and a 4L60e. It's frequently in the wrong gear. I end up locking it in 3rd because it doesn't have the ass to pull OD at city speeds. It could certainly benefit from closer gear ratios and more of them. 

Would it be feasible to replace a 4L60e with a 6L80e? 

It looks like the gear ratios on the 6L80 might benefit a low HP engine. 

6L80e

First gear — 4.027
Second gear — 2.364
Third gear — 1.532
Fourth gear — 1.152
Fifth gear — 0.852
Sixth gear — 0.0667
Reverse — 3.064

VS 4L60. 

1: 3.059.
2: 1.625.
3: 1.00.
4: 0.696.
R: 2.294.

Does the 6L80 have a compatible bell housing? 

How about transmission control? 

Tell me I should just turbocharger it or LS swap it and forget about the transmission. 

I've had a MD tow pig, and a HD motorhome conversion that both needed either more "engine" or more gears. 
The motorhome (at 42K lbs)has a 350hp/1000tq 10L cat, that runs like brand new, in front of an Allison 4 speed slush box.  
Not sure if a 500/1250 C12 or a Allison 6 speed (or 10) would be the better bang to the buck!!!

Different weight class, but interested to see what answers you get. 

How fast is the engine spinning on the highway?

My first thought is regearing the diffs one or two steps lower depending on the current ratio. That will make the ratios in the trans feel closer, and improve acceleration. 

In reply to No Time :

About 2500 at 70. It has 4.56 gears in it now. But it tips the scales at 5400 pounds and the 3.7 is a bit anemic at 240 hp and 250 torque. 

The 6 speed is definitely a much better option. Especially for towing. 
 

I don't know what an HT3 is but the LS with a 6 speed is a yes :)

Powerglide!

In reply to amg_rx7 (Forum Supporter) :

That should have read H3T. Hummers mid size truck. 

This guy.

Honestly it's not that bad but coming from the Touareg with 305 hp and 490 lb-ft of torque it's a little slow. 

I have also kind of looked into this (trading a 4L80E for a 6L90E). The transmission controls for the 6L80/6L90 are dramatically different than the 4L60. They went from the LS1B PCM to the E38 PCM with the transmission swap if I remember right. It seems somewhat like you would need a standalone 6L80/6L90 controller and then segment swap or something for the stock PCM to make the truck think it was a manual. I'm not sure swapping from the earlier PCM to the later PCM is a practical solution, but I didn't seriously look into that because in addition to the transmission differences, they use different tone wheels and whatnot on the engine. I stopped digging because the 4L80 is still working fine and I have too many projects anyway, but the idea is still floating around the back of my head. 

Alternative might be to do some transmission tuning. On my 4L60E-equipped Silverado, there was a lot of room in the stock tables to delay kickdown and allow more throttle, as well as slightly delay upshifts. Stock shift time targets were also very slow in the name of smoothness, speeding things up helped driveability a lot. 

In reply to gearheadE30 :

We reflashed the trans in our WJ Jeep and it made a world of difference. Now it kicks down sooner, one gear at a time instead of holding a gear too long and dropping 2-3. Makes it far more effective over mountain passes.

The 4L60E is actually a really good transmission in general, but the ratios suck donkey balls.

The original 700r4 was introduced in 1982 as a response to several factors, the two biggest of them being A) OD to try and get another couple MPG, and B) a super low first to find some way to make 125 hp V8 wheezers move a 4000-lb Caprice.  The result is a pretty good box with the largest 1-2 ratio change of any automatic transmission to ever come from an OEM.

I detest them for towing.  Not only are they too soft in factory form to really take towing abuse, they suck after 1st.  Getting on the highway with my foot to the floor in my Express van towing a measly 3500 lbs, when it upshifts from 1-2, the acceleration tanks like you just hit a puddle of molasses.  Not to mention, when you're in second and call for acceleration, the drop to first is like you skipped two gears and you're suddenly at redline.

In general, more ratios are needed for applications where torque is limited or when torque is peak-y.  You want to keep the engine swinging in the sweet spot.  When you have an abundance of torque and/or a really flat torque curve, you can get away with fewer ratios.  I remember the hot setup for a while in the local drag race scene was to drop a Caddy 500 in a G-body and back it with a 2-speed powerglide and 3.08s or 3.42s.  They would consistently be faster than a three speed auto because the monster torque peaked at about 1900 and stayed torquey all the way to 4000.  The additional lower gear in a three speed meant that you were either throwing away torque and shifting at 30 mph, or just blowing traction and wasting a run.

I think a 6L80 would be a good choice for you, but unfortunately the Atlas engine bellhousing pattern is unique to just the Atlas engines.  I'm 90% sure that there were no 6L80 transmissions offered behind any Atlas engine, so you might be humped there.  You might be in luck, though with a 6L45 and 6L50, both offered behind Atlas engines.  They are the little brother to the 6L80/6L90.