Transmission swaps that'd fit on the LQ4 6L
 

I have a 2003 H2 been in my family since '06 and the 4l60e was rebuilt in 2015 after a catastrophic failure. I've been trying to research reliable transmission that would be the easiest to drop in on the current motor. My goal is improving the economy of the vehicle as well as some of the performance. I know on later H2 the 6l80 was used and a transmission shop recommended the 6l90 since its a beefer transmission with the added gears to improve some of the highway driving. Any ideas or thoughts would help.

You actually have a 4L65e right now. Anything else will require a good bit of custom work and modifications like computer tuning, custom frame supports, etc. A very small few have converted to a 4L80 (which due to the very different shift patterns designed for HD trucks would likely reduce economy, but is a lot stronger trans) but I'm not sure that any have tried to go to a 6L since those use a totally different electrical system and PCM.

okay, interesting I'm not against large scale customization as this truck has a lot of sentimental value and overall I love how it performs and feels, but the fuel efficiency is what I'm gunning to try and fix so my question to you is do you know what transmission are compatible with the LQ4, the shop I'm looking at doing the work said I could get all the electronics done custom obviously, so I'm trying to find what the best transmission swap while keeping the LQ4 my main goal is to keep the LQ4 til it dies as it's only at 155k rn

the only direct replacement is the 4L65e 4L75e. as mentioned. as far as the LQ4 is concerned if you can bolt a bell housing to it it will run it. but the computer wont. 4L80 /4L90 is a bigger transmission and will need the transmission cross member to get cut and modified also i think there is an issue with the transfer case being clocked the wrong way. you might also have to do a frame lift to clear the transmission in the tunnel. anything with more speeds will need your engine pcm tuned to a manual and then standalone transmission management used to control the shifts.

Has anyone ever tried to put taller gears in the front and rear ends to improve fuel mileage? Just thinking out loud.

You can build the 4l65E to withstand a ton of power, I did it to my old 2005. Its not cheap but its probably the easiest route.

A 4L80E is a pretty easy swap but the gearing is different as compared to the 4l65e
This website has just about everything you need:
https://transmissioncenter.net/

2008 to 2009 H2 had the 6l80E and that swap can also be done.
Look at the harness from PCM of NC here: https://www.pcmofnc.com/product/6l80...ess-generic-3/

Tim