Howdy, new to this forum so I will start with this question. I have 2006 GMC Savana 2500 with the LLY Duramax and the 4L85E tranny. The specs that I have read say it's rated to 460 ft lbs of torque which is a factory tune on the engine. I would like to change to a 6L90 trans rated to 525 ft lbs of torque. Does any body know if it's possible and what changes need to be made. I was told the allison won't fit under a van and that's why for the 6L90 trans. I also want to change the exhaust so which kit do I get, the AFE 49-04001 (http://www.alligatorperformance.com/4904001-atlas-aluminized-exhaust-20012007-duramax-classic-p-10156.html) with the new down pipe? Since I have the 4L85E for now can I still add a programer or do I need to wait for a trans implant? Also on the Tire and Rims issue what would be better a 18 or a 20 inch rim with10 ply tire. Any help would be appreciated.

Other than the extra gears, I would think it would be money ahead to build the 4l85e...

The 4L85E is a stronger trans with many aftermarket parts for it. Any brand, pre-DPF exhaust will work.

doesnt the 6l90 have a better highway gear...less rpms?

Allison will fit, just have to mod the floor between the seats. You would have to get a stand alone computer to control the 6L (also the allison) because it use's different software architecture than the 4L series trans use. If you want extra gears to lower your rpm then go with a gear vendor OD unit. If you want a high torque tranny just get that 4L85 built, they can be be built a heck of a lot stronger than a 6L can be built. Just because GM rates the trans at 460 don't mean that is all it can handle. There are guys on this site that are throwing close to 550# with no problems. It all depends on how you drive:D

I would love more gears in the van. I tend to drive over 75 on the highway to keep with traffic and the rpm's are high.

I would love more gears in the van. I tend to drive over 75 on the highway to keep with traffic and the rpm's are high.

Change your rear end gearing then.

To the OP if you are will to spend the money swap in the ally it has 6 gears and can be built for more than you will put to it.


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Howdy, new to this forum so I will start with this question. I have 2006 GMC Savana 2500 with the LLY Duramax and the 4L85E tranny. The specs that I have read say it's rated to 460 ft lbs of torque which is a factory tune on the engine. I would like to change to a 6L90 trans rated to 525 ft lbs of torque. Does any body know if it's possible and what changes need to be made. I was told the allison won't fit under a van and that's why for the 6L90 trans. I also want to change the exhaust so which kit do I get, the AFE 49-04001 (http://www.alligatorperformance.com/4904001-atlas-aluminized-exhaust-20012007-duramax-classic-p-10156.html) with the new down pipe? Since I have the 4L85E for now can I still add a programer or do I need to wait for a trans implant? Also on the Tire and Rims issue what would be better a 18 or a 20 inch rim with10 ply tire. Any help would be appreciated.
The 6L90 swap is probably only feasible on 2008+ van, which has the newer electrical system. Ben (Duratothemax) knows more about this.

The 6L90's higher rating is not just a specification. It has inherently higher torque capacity than the older 4L85E 4-speed, due to the clutch-to-clutch design, bigger components, etc.
Naturally, some of those who have the 4L85E will disagree with this. There are also 6L90 upgrades to increase the torque holding capacity, although there are many more for the older 4L85E.

As for the Allison vs. 6L90, here's a thread on that: http://www.dieselplace.com/forum/showthread.php?t=442351

I would love more gears in the van. I tend to drive over 75 on the highway to keep with traffic and the rpm's are high.
FYI: the 6L90 top gear ratio is 0.667, compared to 0.75 in the 4L85 and 0.61 in the Allison.
The bigger difference in the 6L90 is on the low end: 4.027 vs. 2.48 in the 4L85. The 6L90's 2nd gear is 2.364.
The 6L90 has a wider overall gear range than the Allison, which has a 1st gear ratio of 3.10.

GM switched to a 3.54 axle ratio in 6L90 DMax vans, but that's only about 5% taller than the 3.73 used previously.

In comparison, just changing from 245/75-16 tires to 265/75-16 tires is approx. a 3.9% change.

I had put 235/85/16 (31.69")tires on my van for a while. Took a major SHIT in MPG. Sold those tires and back to 245/75/16 (30.47") back to normal mpg. I was hoping the 235's would improve MPG bc I thought it would lower the rpm a little on the highway. I don't understand something here.

I had put 235/85/16 (31.69")tires on my van for a while. Took a major SHIT in MPG. Sold those tires and back to 245/75/16 (30.47") back to normal mpg. I was hoping the 235's would improve MPG bc I thought it would lower the rpm a little on the highway. I don't understand something here.

The answer lies in the ratio's Nomad posted. Yes you lowered your rpm on the highway but you slaughtered your efficiency accelerating to highway speed as the torque converter had to do more work with the bigger tires.

GM is bringing out an 8 speed auto soon for RWD applications (lighter duty non diesel I believe) and the extra gears are all at the bottom end.

Back to the original question, if there is a way to do the swap I'd be interested...yes you can build a real solid 4L85...but in stock form the 6L is a tougher transmission and will result in better mileage.

I had put 235/85/16 (31.69") tires on my van for a while. Took a major SHIT in MPG. Sold those tires and back to 245/75/16 (30.47") back to normal mpg. I was hoping the 235's would improve MPG bc I thought it would lower the rpm a little on the highway. I don't understand something here.
Did you correct the speedo/odo for the larger diameter tires?
How long did you run the larger diameter tires? Were you able to do any back-to-back tests under identical conditions?
The taller narrower tires should have produced a tiny improvement in all-highway driving. Is that what your driving consisted of after you changed the tires?
It could also be that they just happened to have more rolling resistance than the tires you replaced. Tread pattern is part of that, but I would have to assume that 235/85/16 were highway tread tires.

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6L90 has lower parasitic loss than either the 4L85 or the Allison (a lot less than the latter). And don't believe people who say there is no loss once the converter is locked up. That reduces the loss but does not eliminate it. That's one reason a manual tranny can have lower loss in top gear crusing conditions, since there is no pump and typically not as much internal lubricant-induced drag either, but auto trannies are getting better in that respect, while manual transmissions aren't improving all that much.
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Piper - According to Ben, the 6L90 swap is pretty straightforward on a 2008+ van but maybe not realistically feasible (though nothing is impossible) on earlier models.
6L90 has its own TCM internally, and it requires the newer GM LAN on 2008+ vans.
6L90 is longer than a 4L85, so it would require shortening the driveshaft, or buying the appropriate 2010+ part.

So your trans TCM is not mounted to the right inner fender? Would the crossmember need to be moved as well?

I don't know about the trans. crossmember.

You might be able to find out by looking at some of the links in the thread I created about the 6L90. http://www.dieselplace.com/forum/showthread.php?t=442351

I went from 245/75/16 cooper atr to Nitto Dura grappler 235/85/16 and ran pretty much all highway for a couple thousand miles. Said screw it and sold those those tires.

I went from 245/75/16 cooper atr to Nitto Dura grappler 235/85/16 and ran pretty much all highway for a couple thousand miles. Said screw it and sold those those tires.

well did you factor in the size difference? with the bigger tire, you would be going faster and traveling further than what your odometer/speedo would say.

I know I corrected it with my programmer.