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2020 M6 HC
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Discussion Starter #1
I hate to beat dead horses, but I would like to get some clear info from people who have actually done it. I am ready to pull the trigger, almost did it today, but there are so many fragmented and conflicting posts that I hesitated.

Could actual users of 3.90’s in M6’s please let me know:

1. Are you happy?

2. Would you do it again?

3. Tell me the good and bad.

4. What was the P/N of the 3.90 diff you installed?

5. Did you install it yourself?

Please hurry, the good weather has arrived and my car has needs!

Thanks!
 

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Challenger SRT Hellcat
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1,784 Posts
Following...I believe we’ve got 3.70’s and if that’s the case I don’t think going to 3:90’s is going to make all that much difference. I think a far more expensive tranny rebuild with a few gear ratio swaps will be required for any substantial improvements.
 

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Challenger SRT Hellcat
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7 Posts
I hate to beat dead horses, but I would like to get some clear info from people who have actually done it. I am ready to pull the trigger, almost did it today, but there are so many fragmented and conflicting posts that I hesitated.

Could actual users of 3.90’s in M6’s please let me know:

1. Are you happy?

2. Would you do it again?

3. Tell me the good and bad.

4. What was the P/N of the 3.90 diff you installed?

5. Did you install it yourself?

Please hurry, the good weather has arrived and my car has needs!

Thanks!
1. I am happy with the 3.90, although not much of a difference.
2. Sure.
3. Not much to tell really, I can say it is a little easier to launch the car on a prepped surface track. No downsides.
4. Cannot remember, did it a few years ago. I believe it the part number comes from the M6 392's.
5. I did install it myself, did the diff brace at the same time.
 

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Charger SRT Hellcat
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105 Posts
Good swap in my opinion. Made the car a little easier to launch and a bit better driving around town/in traffic. I swapped mine in my garage.
 
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I run a 2.98 first gear with the 3.73 rear, 3.90 would be too deep for me. If you want to sell your 3.73 to help pay for the 3.90 I’d be interested in purchasing it.
 

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Challenger SRT Hellcat
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1,328 Posts
i did in my 2015. there isn't much of a difference, like 200rpm? in my mind the car felt better but it's probably not worth it. the CDV delete was a better "mod"
 
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2020 M6 HC
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371 Posts
Discussion Starter #7
I run a 2.98 first gear with the 3.73 rear, 3.90 would be too deep for me. If you want to sell your 3.73 to help pay for the 3.90 I’d be interested in purchasing it.
I've read about your mods and results, sounds like the best way to go. If it were not an almost new car, I would seriously consider that route. Thanks!
 

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Correct me if I'm wrong but wont you also need an unlocked computer to have your speedometer corrected?

Me personally, I would probably go the same route that 16GoManGoHC went and have some gearing changes done in the transmission before swapping the rear gears. Rear is cheaper but not a lot of difference between the stock and 3.90's. I'd really just love a longer 6th gear at .50 OD. I really don't understand why people say 6th gear is useless on these cars as I'm finding that I'm in 6th a lot of the time. At least my car is perfectly happy in 6th gear cruising down the highway or even at 60mph. Our speed limits are 70-75 in most places on the highway so I'd love to drop the RPM a bit when cruising at that speed. That 2.98 first with a .50 6th gear would be awesome for me. A little more acceleration but better cruising RPM on highway.

Here is the ratio in my Viper. I wonder how it would do behind the cat. - 2.66:1, 1.78:1, 1.30:1, 1.00:1, 0.74:1, 0.50:1, :1
 

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Challenger SRT 392
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6,895 Posts
Correct me if I'm wrong but wont you also need an unlocked computer to have your speedometer corrected?
No, from 2015-up the speedo gets it's signal through the ABS sensors so no correction needed.


Sent from my SM-N960U using Tapatalk
 

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2020 M6 HC
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371 Posts
Discussion Starter #10
Me personally, I would probably go the same route that 16GoManGoHC went...
Thanks. I have read many of GMG's posts, he is very serious (more than I will ever be) and has had impressive results making him very helpful. I started this thread to explore the (an) alternative that is less expensive and less invasive and hope to hear back from those who have actually tried it and hear their thoughts. :)
 

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Challenger SRT Hellcat
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795 Posts
1. Are you happy? Yes
2. Would you do it again? Yes
3. Tell me the good and bad. : Dodge Dealership Service Manager said he could install for $500 however would not provide any warranty for the parts or labor and he also said that he would void any and all warranty coverage for my vehicle. That was my last conversation with any Dodge Dealership Service department employee. From that day forward I have done every bit of work and maintenance on my Hellcat myself.
4. What was the P/N of the 3.90 diff you installed? 05038703AG
5. Did you install it yourself? Yes
 
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