Gear swap

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Brian Nickelson

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PEORIA, AZ
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2017
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1500 4 x 4 Laramie 5.7
I have a 2017 1500 4 x 4 with 321 gears and am contemplating the swap to 390 or 392 with a limited slip in the rear. My mechanic won't touch it. Let me know what you choose for gearing and how it changes the MPG and revs. I think I have to do both front and rear but not sure.
 
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McBroom

McBroom

U.S.Marine Veteran Retired 88-2000
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Brian Nickelson said:
I have a 2017 1500 4 x 4 with 321 gears and am contemplating the swap to 390 or 392 with a limited slip in the rear. My mechanic won't touch it. Let me know what you choose for gearing and how it changes the MPG and revs. I think I have to do both front and rear but not sure.
Click to expand...
Yes you must do both front & rear same gears. 3.92s

Black 20 Ram
 
TomB 1269

TomB 1269

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Schenectady NY
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2019 Classic
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5.7 Hemi
McBroom said:
Im thinking about swapping my 2WD 3.21 open diff to a limited slip with either 3.55's or 4.10's all from Matt @moesperformance
Im gonna keep the same tire size(stock).
My question is speedometer and fuel mileage.
I drive like I'm 90 on Sunday. I do watch my fuel mileage very closely.
Currently I avg about 18 city and 23 hwy.

Wife n I plan to buy a 25' TT and want the pulling power. The trailer will be close to max payload when loaded. No the trailer is not up for discussion!!
I'll only be hooked up to it occasionally 2-3 times a month.
Everything else is local city driving.

Black 20 Ram
Click to expand...

Our HP8 transmissions will compensate fairly well and handle the towing nicely even with the 3:21 gear. I just recently towed sons '97 GMC Sonoma extended cab home on U-Haul auto transporter (i.e. full length not tow dolly) other than the squat she did just fine. I estimate the weight of truck and trailer to be 6500lbs if not closer to 7000, as the trucks just over 4000 lbs alone, and the trailer has to push 2000.
 
WY-Dave

WY-Dave

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Brian Nickelson said:
I have a 2017 1500 4 x 4 with 321 gears and am contemplating the swap to 390 or 392 with a limited slip in the rear. My mechanic won't touch it. Let me know what you choose for gearing and how it changes the MPG and revs. I think I have to do both front and rear but not sure.
Click to expand...


Here is a calc for rpms at a given speed, just change the gear numbers to see

https://www.crawlpedia.com/rpm_gear_calculator.htm

For example
1.JPG
2.JPG

@ 65mph
1.JPG
 
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