For whoever’s interested, a 6L90 output carrier assembly is $1100
 from my local Chevy dealer.  The 6L80 and 6L90 have 4.02:1 first gear
 ratios.

 I really don’t know much about automatic transmissions, so I’m not
 completely sure this is the right assembly, but it appears to be the
 only planetary in a 6L90 (or 4L80).  I called up the parts department
 and asked for the first gear planetary, and they were pretty confused
 by that.

 Most of the Chevy and Ford heavy duty trucks (2500HD and Yukon, F250
 and Expedition) from 2007 onward have approximately 4:1 first gear
 ratios.

 https://en.wikipedia.org/wiki/GM_6L80_transmission

 https://en.wikipedia.org/wiki/Ford_6R_transmission

 Somebody who’s more knowledgeable than I am might be able to burrow
 down and get to individual gears rather than the entire output
 carrier in the dealer parts database.

 There are also lots of 3:1 carriers buried in 1999-2011 V8 Grand
 Cherokees down at the junkyard, so that may be of interest to
 somebody if this is a viable way to build a gear reduction drive.

 Dave Klingler

 Good work for finding that Dave. Yes I think the 6L90 rear planet could be made to work
There are two different diameter ring and sun gears so one of the possible
combination will give 4:1.

I have a unit similar to that but a bit smaller I suspect  and it weighs 12 pounds.
The Ford unit we are now using weighs 4 pounds.

In addition to that and when and if the new 3.2L RX9 comes out it will have 20 to 30% more
torque than the 2.6 L  RX8 engine so the need for a 4:1 gear box will be reduced.

 Paul Lamar


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