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Discussion Starter #1
Hello everyone,

I have saved up some money and will be getting either a Diablo Tuner / PCM or 3.90 Diff. I will eventually get both but need advice on which first. My car is a 2018 RT with the 2.6x diff. I have an afe intake, Nitto tires and mufflers. Besides that stock. I’m not trying to be the fastest or anything I know it’s just a 5.7 but I like my car and I want to make it a bit funner.


Thanks for the help!
 

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You will need the tuner for the 3.90 gears to calibrate the speedometer to the change in gear ratio. Plus you can use the tuner to modify other parameters and install a custom tune.

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Discussion Starter #3
You will need the tuner for the 3.90 gears to calibrate the speedometer to the change in gear ratio. Plus you can use the tuner to modify other parameters and install a custom tune.

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Really? I’ve heard there is no need for a tune with a differential swap. I’ve also read the Speedo goes off the front not rear wheels.
I’m really leaning towards the tuner either way.
 

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Discussion Starter #6
Tuner all the way.
I actually just ordered the tuner from Diablo with the PCM exchange program!
I’m excited.

When I went to the Holley page to fill out the form for cross shipment of PCM it’s asking me for the tuner serial number. Does this mean they will send me the tuner first then I fill out the form, then they send the unlocked PCM? Or am I missing something?
 

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Discussion Starter #7
I would like to know how well 3.90 gears help.
Well it would help me in two ways. One I don’t have LSD so that alone would help. Two they would help with acceleration and fun factor. 3.90 is pretty aggressive so I’ve also been looking at the 3.09 LSD.
 

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Discussion Starter #9
I want 3.70 or 3.90. So I may have a deal on 3.09 in near future.
Let me know I’ll be ready to buy before December for sure. I would love 3.70 but they’re 1300 compared to 690 for the 3.90. I can’t justify having my RPM a little lower in 8th gear for 600. Plus I don’t use my car for long commutes.
 

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Discussion Starter #11
No you don't, not on a 2018.



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I just wonder how those down shift will feel without the trans tune lol. I’ve heard of several people that run the 3.90 with no tune at all and no issues but I can imagine those down shifts may be a bit rougher. I’m still probably getting them before December lol
 

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You will need the tuner for the 3.90 gears to calibrate the speedometer to the change in gear ratio. Plus you can use the tuner to modify other parameters and install a custom tune.

Dodge, Jeep & Chrysler Tuning | Jeep Chip | Dodge Programmer & More | Diablo | SCT

DiabloSport - Gasoline and Diesel Tuning Systems
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Not in the 2015+
 

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You know what's odd in the burn out box I have 2 different scenarios. Speedo working and Speedo not working during a burn out. So does it run off front or rear wheel speed sensors.
 

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I just wonder how those down shift will feel without the trans tune lol. I’ve heard of several people that run the 3.90 with no tune at all and no issues but I can imagine those down shifts may be a bit rougher.
Ford and GM use mph set points to determine shifting with the tire size and rear axle ratio as multipliers in the equation. If you change rear axle or tire size your shifting will be off.

Dodge chose to do it differently and in some ways very genius. Dodge shift points happen based on output shaft rpm by gear. So you can change rear axle ratio and tire size but the transmission will always shift at the same engine rpm.
 
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