37's and 3.92

Just looking for any feedback on power loss when going to 37's with 3.92 gears and a 5.7 Hemi?

Evguy1 said:

Just looking for any feedback on power loss when going to 37's with 3.92 gears and a 5.7 Hemi?

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You will definitely notice that your stop light performance has taken a hit. It will be a bit of a **** until you get rolling.

For all of my customers that go with a 37" tall tire and do most of their driving on the street I recommend a 4.56 ratio.

If they are including some off road treks into their driving diet I recommend a 4.88 gear.

Remember, if you have a 4x4 you will need to change the gears in both the front and rear diffs.

Yea those were the numbers I came up with as well. Do you still have to backdate the front diff or has someone started to build gearsets?
I'll start out with the 3.92's and see how it goes. The older I get the less performance I need but I do love the way the 37's fill the wheel openings.

Evguy1 said:

Yea those were the numbers I came up with as well. Do you still have to backdate the front diff or has someone started to build gearsets?
I'll start out with the 3.92's and see how it goes. The older I get the less performance I need but I do love the way the 37's fill the wheel openings.

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If you’re spending the time and effort to regear the truck why not just got to the 4.56’s and only spend the money once?


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At least 4.56's---
Jay

I have BFG KO2's 37x12.5x17 on aftermarket 17x9 wheels with 3.92's and the 6 speed trans. The KO2's in this size are just 69lbs each and my Raceline wheels are 31 lbs each so the combo weights in at about 100lbs per corner. Compare that for example to most Fuel or KMC aftermarket 20" wheels which are in the 40-45lbs range each and something like Nitto MT's at 85lbs each in 37x12.5x20 and you're looking at 130-135lbs per corner with those. Big difference in unspsrung weight for everything from acceleration to braking.

My truck weights in at 6200lbs everyday without me, 6395lbs with me inside and around 7400-7500 lbs fully loaded for familly weekend/vacation fun.
I don't tow anything and l feel the height and weight of the 37's worst as i have the 6 speed trans especially when loaded. I'm sure it will be better if i had the 8 speed. In retrospect I can still get a quick bark from the rear from a dead stop if i punch her lol, but in no way shape or form she's got the peppy feel she had with the 33's. I also have to use the tow button when i'm going uphills and there is also a penalty on the MPG department more accentuated during city driving. I don't have the 4 cyl stuff either and i can only imagine those driving 37's with that system enabled, Yuck.

I did 37's looking for a bit more extra clearance offroad and also because i wanted to be different, everybody around here run 37's on a 6" lift. I'm not into those lifts as i really like and enjoy a low COG and as long as nothing rubs offroading life is good and looks is exactly what i want. But lately i've been rethinking going back to 35's in the future mostly for MPG. I'm driving a lot more in the city when the truck is not in the dirt and trust me the MPG penalty adds up quickly to the wallet when a full tank is around $80 every 2 days.

Regarding gearing, originally i wanted to regear to 4.56 and put a locker in the rear. I came from a Jeep Wrangler that i was using as DD and heavy rock crawling chit so I thought this was something that was going to cost me $2500 OTD to do this to the RAM. Why did i think that ? I paid $1000 OTD to have my Jeep Wrangler regeared to 4.56's right in my garage with me watching then extra $1500 to have an Eaton elocker rear and Detroit truetrac in the front on a different day. I could have saved about $500 in labor doing everything at once since the same guys did the job.

Came to find out doing after a while the same on a 1500 4x4 will be a lot more expensive, not counting there is no front locker or truetrac front available. You have to put parts from the 2011 or older 4th gen trucks, no rear locker for the ZF diff either so in the back you want the older 4th gen diff with the older axels. Then you can add a locker for the old diffs.... a lot of money and it's still an IFS front without a front traction device, thanks but no thanks. I'm just glad having learned a little lesson from buying a Jeep with 3.21's: always go with the factory option that offers max gearing and part of the reason i looked at only RAMs with 3.92's. Didn't know about the 6 speed vs 8 speed differences until recently that i bought a 2014 Durango with the 8 speed. i'd have skipped the 6 speed all together as well.. live to learn right ?

Hoping my 5 min reading will give you a better idea of what to expect with 37's. Is not super bad in the power department with 3.92's and the 6 speed trans and a Hemi as long as you're not trying to race Tundras and the like at every stop light or do hwy punches from a roll hehehe. City MPG will suck. I do get 15-16mpg at around 70mph fw only. The experience will improve a ton with 4.56's but in my case is cheaper to go back to 35's.

Have fun.

Thanks for the epic reply ram1500rsm!
I'm installing Falcon Wildpeaks and they weight 82 lbs on stock wheels. I also did not want a lift other than a 1.5" body lift.
I'm trying to keep weight down on anything I add. VICE designs is building me an aluminum version of their bumper. https://www.vicedesign.ca/bumpers.
The Hemi is coming out and a Cummins ISB 170/Allison 1000 is going in. I have been running the same engine in my Dakota for the last 7 years and tow our 5th wheel all over North America.
Not a lot of power 225/450 but a fair bit of torque and all done by 2800 RPM.
I have always been a small truck guy so this is my first full size build. Is there ever a LOT of room in a full size truck!

Michael said:

If you’re spending the time and effort to regear the truck why not just got to the 4.56’s and only spend the money once?

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The truck came stock with the 3.92's. It took me a while to find a truck that ticked all the box's for me especially the gears and limited slip diff but this one is close.

Evguy1 said:

The truck came stock with the 3.92's. It took me a while to find a truck that ticked all the box's for me especially the gears and limited slip diff but this one is close.

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I thought you were regearing to 3.92’s before trying lower gears.


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I was lucky enough to find a truck with them. I'll see how they do with the Cummins and Allison 6 speed before changing gears.

I have the 545RFE and 35’s with 4.56’s and a 6” lift. Effectively around a 3.91 ish hear ratio. Truck feels like it has no lift at all.

If I go 37’s I don’t want to ruin the trans, but also was thinking of swapping to the G56 manual that is in the 2500’s.

All in all 4.56 with a 5 or 6 speed is great for making the truck feel like stock again.

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Now that I have the 8hp70 swap the 4.56 gears are a bit high . If I slowly accelerate from 0-45 I can go through the first 4 gears supper quick. My 4.56's I believe are starting to whine at about 50-60mph, so I need to get them checked out. 4.56's are not an OEM cut so I know they are a bit noisy, but not like they are right now.

I was wondering if anyone is on 37's with 3.9's and the 8hp70? If I can go back to 3.92's or even 4.10's I can get an OEM cut and they may last a lot longer. I don't want to sacrifice the performance though.

Noise has nothing to do with cut. My 4.56 Motive Gears are as quiet as the OEM ( and it was my first time ever doing gears BTW ) . Noise is from an incorrect pinion depth or just bad install.

Get FRP to do a custom trans tune with the 4.56 gears . A better TQ is also needed to get the full performance out of it.

RAMTRPR said:

Now that I have the 8hp70 swap the 4.56 gears are a bit high . If I slowly accelerate from 0-45 I can go through the first 4 gears supper quick. My 4.56's I believe are starting to whine at about 50-60mph, so I need to get them checked out. 4.56's are not an OEM cut so I know they are a bit noisy, but not like they are right now.

I was wondering if anyone is on 37's with 3.9's and the 8hp70? If I can go back to 3.92's or even 4.10's I can get an OEM cut and they may last a lot longer. I don't want to sacrifice the performance though.

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I have 3.92 and 35s. If I’m cruising at 50 mph my truck will be in 8th gear. I wouldn’t mind 4:10s in my truck but 3:92 isn’t far from that so no point for me to swap.

crazykid1994 said:

I have 3.92 and 35s. If I’m cruising at 50 mph my truck will be in 8th gear. I wouldn’t mind 4:10s in my truck but 3:92 isn’t far from that so no point for me to swap.

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Sorry quoted wrong person......LoL

RAMTRPR said:

Now that I have the 8hp70 swap the 4.56 gears are a bit high . If I slowly accelerate from 0-45 I can go through the first 4 gears supper quick. My 4.56's I believe are starting to whine at about 50-60mph, so I need to get them checked out. 4.56's are not an OEM cut so I know they are a bit noisy, but not like they are right now.

I was wondering if anyone is on 37's with 3.9's and the 8hp70? If I can go back to 3.92's or even 4.10's I can get an OEM cut and they may last a lot longer. I don't want to sacrifice the performance though.

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Your actual gear ratio is 4.07 when running 37's with a .456 gear
So you are fairly close to the stock 33's with a 3.92 gear
Actual gear is 3.50 on 37's with 3.92 gear
Actual gear is 3.66 on 37's with 4.10 gear
That 8hp70 will handle them all in the lower gears in transmission but with those large heavy tires would be best with the .456 in 8th gear.