Gearing a 14' for 37's

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RubberFrog

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I thought about rehearing to 4.11's, but figured the 3.92's would work. It's more money and perhaps more down time. There was a math equation that shows

33/33= 1 X 3.21 (Stock setup)
33/37=.89189 X 3.21 would net me 2.86 (37'sw/ Stock gears)
33/37 = .89189 X 3.92 would net me 3.49 so that's what makes me think 3.92's should be fine.

Does the math work out. Am I correct in my thinking? I don't hate the 3.21's. I know the 4.11 or 4.56 would make it feel more peppy, but is it totally necessary?

The math (I haven't double checked it) does not take into account the extra rotational mass of the larger tire. Using a simple calculator, it assumes tire mass remains constant. 37s are heavy. It's like trying to haul ass with a trailer.
 

Rken1288

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I have 3.21s with the 8 speed and stock size tires and I never get 20 mpg at 80. On the highway our speed limit is 70 so I set it on cruise at 75-80 and on the evic it shows around 18.5 usually. 19 would be on the high side. I'm sure if i hand calculated it it would be worse. How are you guys and gals getting such good mileage even with bigger tires? I'd like to know your secrets.



They might be exaggerating just a wee bit.....[emoji12]


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OUMX117

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I have 7" lift 35s with 3.21 . I put hemi fever tune it moves just fine mpg is good to get about 20 mpg at 80

Riiiiiight.... 3.6 pentastar going downhill? Not going to happen in a hemi.
 

Ram444

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I have 3.21s with the 8 speed and stock size tires and I never get 20 mpg at 80. On the highway our speed limit is 70 so I set it on cruise at 75-80 and on the evic it shows around 18.5 usually. 19 would be on the high side. I'm sure if i hand calculated it it would be worse. How are you guys and gals getting such good mileage even with bigger tires? I'd like to know your secrets.
Their secret is good drugs and a nice set of hip waders to improve your mileage every time.
 

xrsman

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I have 7" lift 35s with 3.21 . I put hemi fever tune it moves just fine mpg is good to get about 20 mpg at 80
Don't write **** like this. People will think you're serious and be disappointed when they find its actually half that. What a stupid thing to say.

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xrsman

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Decrease gas mileage further?
You'll get better mileage with a 3.92 rear end running those tires than your current one. You'll definitely want to get a tuner though, those 37's will kill your power!

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Pmexpert

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I've switched to 4.56 as oem 3.92 were not enough for 37'' tires. So truck runs perfect especially with JG tune(93 octane). Fuel consumption...hmm. it looks like it's similar to initial one before installing lift kit and 37'' tires.
 
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deangtp

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Well, I decided to give the 3.92's a try. Based on the ease of switching out front diff and rear axles. I got 2015 units with 9k miles on them. Front 3.92 unit and a rear 3.92 with the anti-spin. I should be able to switch out the units fairly quick. If it's a dog, maybe next summer I'll move up a ratio or two. But this'll have to do for now.
 

RAM_Designs

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I've switched to 4.56 as oem 3.92 were not enough for 37'' tires. So truck runs perfect especially with JG tune(93 octane). Fuel consumption...hmm. it looks like it's similar to initial one before installing lift kit and 37'' tires.

What are your rpm's like crusing at 70 mph with the 4.56's? With my factory 3.73's I don't hit 2k rpm til around 80-81mph.
 

Pmexpert

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What are your rpm's like crusing at 70 mph with the 4.56's? With my factory 3.73's I don't hit 2k rpm til around 80-81mph.

Will check that tomorrow. As I have 8spd transmission, don't think PRM on that speed will be higher than 2K
 
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