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regearing required to run 33s with 3.7L

10K views 20 replies 6 participants last post by  dudefromflorida 
#1 ·
Hey guys,

Im trying to get some information on regearing my 07 wk 3.7L so I can run 33" tires. So far I know that the lowest I can go is 3.73 ratio most likely from a wk or xk. Or can I get the ring an pinion from another site as long as its made for a dana 44 axle? Any help or insight on this is greatly appreciated.
 
#2 ·
there are a few people running 33's on their 3.7 with stock gears and no problems. look into getting the hypertech max programmer to recalculate tire size.

my next set of tires will be 33's, no plan to regear for the time being.
 
#3 ·
from what Ive heard adding that big of a tire with our current gearing and engine size will put a huge strain on the engine and transmission and cause a severe loss of power and a very short life of the axles and transmission. Another forum had a guy that swapped the rear gearing for 3.73 and put a new front differential from a commander in that had 3.73 ratio. Then he used the hypertech programer to set up the transmission to recognize the new ratio and tire size.
Im trying to find out now if our rear axle is a dana44 or the chrysler 8.25".
 
#4 ·
You've got the corporate 8.25. The Dana was only used in the SRT8. You'll find no problems getting the rear R&P in the aftermarket in different ratios, its the front thats not available. So you are stuck with stock ratios, either 3.55 or 3.73.

I've been running 33s with the 3.7L for over 40,000 miles and have had zero issues that you mention with the axles/trans. Yes, there is a slight loss of power and I would definitely not characterize it as "severe." I've even added armor and a rack and have no severe power loss issues. If you are that concerned about power loss, then you need to be looking at trading your Jeep in for a hemi.
 
#5 ·
Thanks Mr. Boh, do you happen to know if the pinion flange is different on the 06-07's for the corporate 8.25?
Im trying to gain as much insight on this before I continue with any purchases or modding so Im basically going by what others have told me. But since you are actually running the set up, how bad is your gas mileage now and do you have any whining coming from the transmission? Did you use a hypertech programmer to set the wheel size and speedo recalibration?

I have already found a used front differential that is 3.73 ratio, and after calculating the cost of a new ring & pinion and other parts and tools ill need for a ratio change for the rear i could spend 50 more dollars and just get a used rear axle i found with 3.73 ratio. Do you think it would be worth it to swap them out or is it just a waste of money? Im wondering how big of a difference it would make just by the ratio change
 
#6 ·
Sorry, can't answer your question on the pinion flange. My plan was to always swap 2005 to 2005 so that I wouldn't have to worry about any design change issues (if any). After all my mods...lift, tires, armor, rack, I'm down about 2 mpg on the highway. No whining from the trans. Don't have the Hypertech so my speedo is still off. However, I do plan on getting it soon as going up the mountains in WV is painful....the new gears and add hp will be welcomed.

As far as swapping the R&P or R&Ring the front diff and rear axle, I guess I would have to ask if you are doing the work yourself and if so how comfortable you are doing it. To me, R&Ring an axle is easier (assuming nothing has to be done to it) than shimming up a new R&P set. But that's just me.
 
#7 ·
I do the work myself on my jeep and Ive never done a ring and pinion change so im really not wanting to mess up my daily driver and would prefer to just put in a different axle. Would you happen to know what the gear ratio change actually does? will it increase torque to push the bigger tires and ease engine strain? Thanks for the help
 
#8 ·
im assuming the other forum you read about the gear swap was jeep forum...if so, talk to aj (ajeepzj) he also has 33's on his 3.7. there are a few others out there but the only other one i can think of off the top of my head is on here, nick (gcl06), he has a superlift on 33's...
 
#12 ·
Hmm Im thinking it will just be easier to get they hypertech programmer to set the new tire size and if I run into problems I will change the gear ratio then. Thanks for all the help guys.
 
#13 ·
Well I havnent posted on here in a long time since I ran into LOTS of issues with my jeep. To sum it up:
1. Have a SuperLift stacked up to equal 6" all around lift
2. Was running 33" Nitto MudGrapplers
3. Pinion Snapped right at the flange connection to the drive shaft at the rear axle on the highway after about 4 months of running it like this
4. Decided to Swap both front and rear axles to a 3.73 ratio
5. My transmission was jacked from the drive shaft breaking off so I swapped the transmission as well
6. everything is installed and I CANNOT GET MY JEEP TO GET OUT OF LIMP MODE

Ive used the hypertech programmer to account for the gear swap........Did not work. im still getting the code of Gear ratio error in first.
Ill erase the code with my autoenginuity scan tool...reprogram with the hypertech and my jeep will pop right back into limp mode right after I drive about 100 feet.

If anyone has any ideas to how I can fix this PLEASE HELP. Ive been without a car and bumming rides for 2 months now. Im sure its a computer issue from regearing but I have no idea how to fix it.
 
#14 ·
Ive read around and there was a somewhat similar situation with fishinbama's jeep on jeepforum and he was able to get ahold of Blown7 from the SRT8 forum to fix the issue with the ECU, PCM, and TCU. I have not been able to get ahold of either of them though
 
#15 ·
Might want to try and get a TCM from a wrecked SRT8 as that has the same trans as the 3.7L and is already programmed for 3.73.

Also, did you contact Hypertech and let them know what's going on? That their product does not work as advertised to correct for gear changes. They might have a software update or provide you with additional guidance.
 
#16 ·
They said there is nothing on their end they can do that it must be an issue with my car. I may try and find a tcm from a wrecked SRT8 and see if that works. Do you happen to know if the dealer can just reprogram my TCM to accept the 3.73? Thanks for the help
 
#17 ·
While reading this, I was going to recommend exactly what you just said.
Call the dealer and explain the re gearing. They may be able to just re-flash your TCM or steer you towards a TCM that will work.

I will ask a few friends that I know locally that are pretty big Jeepers and see what they have to say.
 
#18 ·
My understanding is that the dealer can't do it. Everything is tied to your VIN. Since 3.73 wasn't an option with the 3.7L/NAG1 the dealer won't be able to program it since its not an option. 3.55 was optional with the 3.7L/NAG1 so they might be able to do that. But again, from what I've heard, everything is tied back to your VIN so if your Jeep wasn't built that way, the dealer has little options available to them in changing these types of parameters. You need a man like Blown7.

Sounds like a cop-out to me on Hypertech's part.
 
#19 ·
I sent a text out to a few guys. You only option from what I have been told is a reprogram on the TCM. Look up blown7.. or find someone you know in the stealership world and see what they can do. If you just call up there they will tell you they cannot do anything. You need to locate a person who you actually know, and who actually knows Jeep.
 
#20 ·
Thanks so much man I appreciate the help. I'm in the process of asking dealers from other areas than my current city (erie, pa) since they were no help at all.
 
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