Changing 6th Gear Ratio 6l90

[macguy]

On the 6L90 the 6th gear ratio is .67:1 Can this be changed to .72:1? Reason I ask is that vehicle searches for right gearing (down shifts up hill at 70mph), which I know is placing stress on transmission. I know if I had 4.10 axle this would happen less, but since I only have 3.73 axle, I'm wondering if a change can be made easier to transmission gearing.

[Stevens03]

Probably easier to swap your axle gears

[Anthonyb]

I am going through the same thing. I have a 08 2500HD with the 6.0 with the 6l90 with 3.73. I just upsized my tires and I am considering going to 4.11. I made this chart, it might be useful for you.

 

 

[TorkFaze]

I won't go into how planetary gearsets work and how they're 'stacked' in most modern auto transmissions, but suffice to say that you can almost NEVER change one gear ratio in a modern auto trans. Because of how the gearsets are stacked, most specific gear ratios are a function of 2 different gear sets at the same time. If someone made the parts and they were affordable, you'd end up changing more than just the 6th gear ratio and you may not like the results of the other ratio(s) that change.

 

That said, if the trans downshifts and holds 5th on the grade you're talking about for a while, you're not hurting the trans at all. If the trans shifts back and forth quickly (less than 15 seconds or so in each gear) AND it does this a bunch of times (10+) then it can be hard on clutches.

 

If it's just annoying to you, your cheapest option is to pull the shifter in to M5 mode and run the grade in 5th. However, unless you're moving your foot around alot (and lifting out a bunch) after it downshifts, it should hold 5th until after the grade flattens.

 

See the service bulletin that MS3Dale posted in http://www.gm-trucks.com/forums/index.php?...st&p=982121 for a brief description of what should happen (shift stabilization)

 

 

Hope this helps!

 

TF

[macguy]

I won't go into how planetary gearsets work and how they're 'stacked' in most modern auto transmissions, but suffice to say that you can almost NEVER change one gear ratio in a modern auto trans. Because of how the gearsets are stacked, most specific gear ratios are a function of 2 different gear sets at the same time. If someone made the parts and they were affordable, you'd end up changing more than just the 6th gear ratio and you may not like the results of the other ratio(s) that change.

 

That said, if the trans downshifts and holds 5th on the grade you're talking about for a while, you're not hurting the trans at all. If the trans shifts back and forth quickly (less than 15 seconds or so in each gear) AND it does this a bunch of times (10+) then it can be hard on clutches.

 

If it's just annoying to you, your cheapest option is to pull the shifter in to M5 mode and run the grade in 5th. However, unless you're moving your foot around alot (and lifting out a bunch) after it downshifts, it should hold 5th until after the grade flattens.

 

See the service bulletin that MS3Dale posted in http://www.gm-trucks.com/forums/index.php?...st&p=982121 for a brief description of what should happen (shift stabilization)

 

 

Hope this helps!

 

TF

 

I've tried the manual mode, but it still down shifts on its own even in M5.

[TorkFaze]

I've tried the manual mode, but it still down shifts on its own even in M5.

 

 

If it's downshifting when you've pulled it into M5 then it's going into 4th or lower and likely needs to. (Must be a pretty good hill if you're empty and getting this at 70 mph!)

 

That said, a different 6th gear ratio wouldn't help (even if you could do it) if 5th gear at ~0.85 isn't enough ratio. A 4.10 instead of a 3.73 axle will help....it's kinda like putting 6th gear at ~.74 and 5th at .93 with your 3.73 (and other ratios move similarly)....but if your 3.73 needs 4th gear (which is 1.15 or so), a 4.10 axle might need it too.

 

One more thing....using manual mode won't limit downshifts, but it will limit upshifts to help keep the trans from shifting back and forth between gears if it's doing that too frequently for your liking.

[Mcguinness]

I am going through the same thing. I have a 08 2500HD with the 6.0 with the 6l90 with 3.73. I just upsized my tires and I am considering going to 4.11. I made this chart, it might be useful for you.

 

 

 

Nice job on the chart

I also had been wondering about this.

I just changed from 245 75 16 to 265 70 717.

And with it have noticed a slight change in the shifting and

rpm's.

 

Thanks for the info!!!

[[email protected]]

two ideas: 1). speed up to 80 or so! that may get you 200 more RPM's into the power band, and should be fine...of course that may not work to good on a us hwy w/ 55mph speed limit and plenty of state patrol presence.

2). use 5 th gear and save 6th for flat terrain or higher speeds. According to that chart at 60-70 you should stay around 2000- 2400 rpm's depending on tire heights. that's reasonable and should get you out of downshift problems,albeit not as efficient as 6th BUT it'll save a pretty good sized parts/labor bill for that rear gear swap/tranny gear change.

Q. does anyone think being in 4cyl mode have anything to do with this???

[Rat4go]

Q. does anyone think being in 4cyl mode have anything to do with this???

 

 

There are no vehicles that I'm aware of that have AFM and a 6L90 trans (6L90 is used for HD trucks, 6L80 is for 1500 trucks).

 

Also, my experience with AFM is that it always bails out of 4 cyl mode BEFORE the trans would think about downshifting.

[macguy]

Q. does anyone think being in 4cyl mode have anything to do with this???

 

 

There are no vehicles that I'm aware of that have AFM and a 6L90 trans (6L90 is used for HD trucks, 6L80 is for 1500 trucks).

 

Also, my experience with AFM is that it always bails out of 4 cyl mode BEFORE the trans would think about downshifting.

 

There is some sort of torque management, as there is a slight delay before downshifting similar to responce of an AFM setup.