L86 to L8T in '19 Yukon XL Denali

[Marky Dissod]

4L60's 3rd: 1.000 . . . . . . . . . . . . . . . . . . . 4L60's 4th: 0.696 . . . (5 speed manuals, woefully underappreciated)
6L80's 4th: 1.152 . . . 6L80's 5th: 0.852 . . . 6L80's 6th: 0.667 . . . (too bad 6L80 lacks a direct drive gear)
8L90's 6th: 1.000 . . . 8L90's 7th: 0.845 . . . 8L90's 8th: 0.652 . . .
10L80 7th: 1.000 . . . 10L80 8th: 0.854 . . . 10L80 9th: 0.689 . . . 10L80 10th: 0.636

In other words, 4L60's last gear ≈ 6L80's last gear ≈ 8L90's last gear ≈ 10L80's 9th gear.
In other words, only 10L80's 10th gear meaningfully improves MpGs -
in conjunction with direct injection at several thousand psi, 5W20 motor oil, newer ATF,
and Engine Confusion, which many of you are forced to disable by avoiding 10th entirely,
if you can't disable it with a plug-in.

 

[Bigburb3500]

The next-to-final and final gears of the 6L80 & 8L90 are pretty damn close to the same,
and 8th and 9th of the 10L80 are pretty damn close to the same too.
If you can take advantage of 10th (can keep the TCC locked) and downshift to 9th as rarely as possible,
yes, it can be worth a few MpG.

However, if you cruise at a high enough road speed, the 10L80 MIGHT vacillate between 9th & 10th?
Which I guess would still be better than just 9th?

Oh, by the way:
more to come ...

They might be close but the truck cruises so close to the sweet spot. I’m high jacking this thread so I’m going to stop, but we can continue this back on the 3500 thread.

 

[CMoore711]

The more I am seeing these swaps, the more I want to intentionally find a K2X with a bad motor specifically to do this...

I know. That’s a diamond in the rough, but it would be ideal to find a K2XX with lifter failure that someone wants to dump instead of fix.

13 MPG isn’t great, but it’s not terrible. That being said 13 MPG of regular unleaded 87 octane is leaps and bounds better than 13 MPG on 91/93 Premium fuel.

I’m sure your mpg will get better as you get the tune dialed in.