New gearing???

[Panthers65]

Well I personally think my Rubicon has plenty of power . I have pulled my big pop up and the bed of the jeep loaded down with camp gear and firewood up steep terrain dirt road even and it does fine.
Let me ask a question just how high of a gear do you want while ypur towing 3000 pounds up a steep dirt road ? 6th, 7th ,8th.. I will tell you what I think I think everyone just has different views of performance and power and how fast they want to go or how high of a gear they think they need to pull a load in
. Cheers.

That's exactly the point, it all depends on what you want out of the vehicle. I've worked on moon buggies, 4cy crawlers, big block rock bouncers, all the way to KOH and Baja trucks, and there is no 1 answer fits everything. I had a 80HP Samurai-based rock crawler on 40's that could idle up a vertical wall thanks to its gearing, but had a top speed of about 30mph, Had I had the factory gearing in that buggy I would be putting in a new clutch after every weekend trip. Worked on 1000HP 1ton built rigs running 44's on 4.10's that had been in service for years, no problems. There's a reason the old SM465 transmissions with the granny first gear were such monsters and everyone loved them.

Lower (numerically higher gears) reduce stress on the transmission in exchange for engine RPMs. The exact same Jeep on 40's will have less strain on it's transmission with 5.13's vs 4.10's, but it will run at a higher RPM with the exact same speed/transmission gear.

In your situation, pulling a trailer up a steep dirt hill, what would be BEST for the Jeep would be to drop it into 4lo and sit at about 2K all the way up the hill, that would put the least amount of stress on the Jeep, resulting in the lowest amount of heat in the transmission and keep the engine in the most efficient torque range. Depending on where you are, that may or may not be practical. Most people don't want to idle around everywhere they go, so compromising and trying to minimize stress/heat while having the jeep perform how you want it to

On small 37's (Closer to 35.75) 5.13's, and Jeeps .67 final OD gear; I'm running around 2400RPM at 75MPH. This is perfectly comfortable to me and as fast as I really have any desire to drive my Jeep. I really enjoy the deeper gears around town and at lower speeds, I feel like it reduces stress and heat on the Jeep in stop and go traffic and especially when I'm pulling my boat (5K+ when loaded up). It also puts the Torque right in line with the torque curve for the 3.6 as far as efficiency goes.

If my needs/driving style was different, I definitely would have considered a different gear ratio. Then again if I wanted a vehicle where I would primarily be doing 80+ MPH, a Jeep would probably be my last choice. At the end of the day we are driving bricks on wheels. No gearing will make up for the piss poor aerodynamics of a Jeep windshield.

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[Zachanadandy]

That's exactly the point, it all depends on what you want out of the vehicle. I've worked on moon buggies, 4cy crawlers, big block rock bouncers, all the way to KOH and Baja trucks, and there is no 1 answer fits everything. I had a 80HP Samurai-based rock crawler on 40's that could idle up a vertical wall thanks to its gearing, but had a top speed of about 30mph, Had I had the factory gearing in that buggy I would be putting in a new clutch after every weekend trip. Worked on 1000HP 1ton built rigs running 44's on 4.10's that had been in service for years, no problems. There's a reason the old SM465 transmissions with the granny first gear were such monsters and everyone loved them.

Lower (numerically higher gears) reduce stress on the transmission in exchange for engine RPMs. The exact same Jeep on 40's will have less strain on it's transmission with 5.13's vs 4.10's, but it will run at a higher RPM with the exact same speed/transmission gear.

In your situation, pulling a trailer up a steep dirt hill, what would be BEST for the Jeep would be to drop it into 4lo and sit at about 2K all the way up the hill, that would put the least amount of stress on the Jeep, resulting in the lowest amount of heat in the transmission and keep the engine in the most efficient torque range. Depending on where you are, that may or may not be practical. Most people don't want to idle around everywhere they go, so compromising and trying to minimize stress/heat while having the jeep perform how you want it to

On small 37's (Closer to 35.75) 5.13's, and Jeeps .67 final OD gear; I'm running around 2400RPM at 75MPH. This is perfectly comfortable to me and as fast as I really have any desire to drive my Jeep. I really enjoy the deeper gears around town and at lower speeds, I feel like it reduces stress and heat on the Jeep in stop and go traffic and especially when I'm pulling my boat (5K+ when loaded up). It also puts the Torque right in line with the torque curve for the 3.6 as far as efficiency goes.

If my needs/driving style was different, I definitely would have considered a different gear ratio. Then again if I wanted a vehicle where I would primarily be doing 80+ MPH, a Jeep would probably be my last choice. At the end of the day we are driving bricks on wheels. No gearing will make up for the piss poor aerodynamics of a Jeep windshield.

2k rpms is the most efficient torque range for the 3.6L.... that doesn't even hit peak torque until nearly double that. There's a reason why it downshifts and puts the rpm up around 6k, because that's where the engine makes it's power. If that worked your argument for gearing would be backwards and we'd have 5 overdrive gears to idle around everywhere, but it doesn't because you need the power to do the work.

 

[Panthers65]

2k rpms is the most efficient torque range for the 3.6L.... that doesn't even hit peak torque until nearly double that. There's a reason why it downshifts and puts the rpm up around 6k, because that's where the engine makes it's power. If that worked your argument for gearing would be backwards and we'd have 5 overdrive gears to issue around everywhere, but it doesn't because you need the power to do the work.

Efficient isn't the same thing as peak... I don't have the desire to explain it tonight, but if you need peak torque all the time it perfectly exemplifies why you need deeper gear.

 

[Zachanadandy]

Efficient isn't the same thing as peak... I don't have the desire to explain it tonight, but if you need peak torque all the time it perfectly exemplifies why you need deeper gear.

But idling or rpms well below any power output isn't efficient at doing work. It is the most efficient under low power requirements which is the point of 2 overdrives and the very reason not to gear too deep. Downshift to the power band when you need it and run 8th gear only under low power needs to maximize efficiency. Best of both worlds. If I'm climbing the grade from baker towards Vegas at 85mph, the Jeep downshifts and brings in the power needed to push this lifted brick on wheels up hill for 20+ miles. Headed the other way, it's 2k rpms at 85mph in 8th gear as it doesn't need the power. Gearing to 5.13s wouldn't make the uphill any better but it would be like manually shifting into 7th and spinning 500 more rpms than I need for no reason.

 

[Panthers65]

But idling or rpms well below any power output isn't efficient at doing work. It is the most efficient under low power requirements which is the point of 2 overdrives and the very reason not to gear too deep. Downshift to the power band when you need it and run 8th gear only under low power needs to maximize efficiency. Best of both worlds. If I'm climbing the grade from baker towards Vegas at 85mph, the Jeep downshifts and brings in the power needed to push this lifted brick on wheels up hill for 20+ miles. Headed the other way, it's 2k rpms at 85mph in 8th gear as it doesn't need the power. Gearing to 5.13s wouldn't make the uphill any better but it would be like manually shifting into 7th and spinning 500 more rpms than I need for no reason.

Man I don't know how else to explain it to you, you're not getting it and I don't have the crayons to explain it another way.

If it works for you that's great, have fun. There's a reason 99% of performance lifted vehicles match the gearing the way they do. You keep bring up 85MPH, but that's simple not what the majority of Jeep owners are doing with their vehicles.

I'm done explaining it, you want to run 3.55's and 40's on your Jeep have fun, someone gotta keep the transmission rebuilders in business. I know I know you've got a whole 25K on your jeep and haven't burnt the transmission up yet...that's great.

 

[Zachanadandy]

Man I don't know how else to explain it to you, you're not getting it and I don't have the crayons to explain it another way.

If it works for you that's great, have fun. There's a reason 99% of performance lifted vehicles match the gearing the way they do. You keep bring up 85MPH, but that's simple not what the majority of Jeep owners are doing with their vehicles.

I'm done explaining it, you want to run 3.55's and 40's on your Jeep have fun, someone gotta keep the transmission rebuilders in business. I know I know you've got a whole 25K on your jeep and haven't burnt the transmission up yet...that's great.

Transmissions burn up from running in 7th instead of 8th? These aren't the wide ratio 3-4 speed autos of yesteryear. I'll let you know if it needs a rebuild, but I'll bet the 3.6L eats the rockers 1st. Of course in your crayon math that will be caused by axle gearing magic too.

 

[Stan H]

I like crayons just kidding I bow out. Have fun fellas.

 

[djthumper]

Thank you. I have not built up a Jeep in about 30yrs. I'm sure some things have changed since then.

In Vegas you will likely want to regear due to our terrain just in town is up and down.

 

[Blade1668]

Hi, Y'all. I'm getting ready to install a 2.5" lift on my Rubicon. I was told that 37's would fit just fine. However, I would have to regear the Jeep or it would struggle on the freeway from lack of power because of the size tire. Was I given the correct information? Or, was this person being overly cautious? Any information would be great to clear this up before I order any parts. Thank ya much.

Experience I've had in older Jeeps. My MJ's and XJ's came with 3:55/3:54 gears with 27-29in tires (215-235) sizes. I re-geared my lifted XJ to 4:56 and run 33-35in tires it actually ran same with it matching the speedometer. I could "light up the tires" on pavement or slide it sideways or whatever stupidity. It also improved my gas mileage when and if not doing the before mentioned stuff or off-roading. I-state driving at 70-75 mph pulling 20 or so mpg's. I did the gears for off-roading and added lockers.
My 05LJ 3:73 is running heavy ass (old) 33MT's not good mpg's but speedometer is fairly accurate it was better with 31-32 tires it came from factory with 30in tires.
Now on my Sport S Max-Tow 4:10 factory 245x70r17 now running heavy 33x10.5r17 I've lost about 10 mpg's (road driving) but I'm not concerned about switching to 35-37in tires needing needing a re-gear. "the tires I have were closer to a 34 in when new than 33." Most trucks/Jeeps with larger than factory haven't been re-geared. Main use of the vehicle makes the reason to re-gear. Towing or heavy duty use? Pavement just driving it if you keep tire weight and rims in check not as much of a issue on mostly flat terrain. Off-roading some of the same if you air down. If re-gearing then adding lockers is really a must do since you are already changing the gears. IMHO. I re-geared my XJ for off-roading the added bonus of highway wasn't a concern or even expected. It's been from Key West to Bloomington IL and all over the S.E. USA with about 70k off-roading. My LJ has been from St. Petersburg FL. to Springfield, MO. and Texas. A few inches taller tires might be a concern when hauling stuff on trailer or pulling a heavy T.T. up and down mountains regularly. My LJ was my tow vehicle for my 16ft Scamp T.T. with 31 MT. doing many miles as 4 sets of tires on the Scamp T.T. in 10 years.

 

[AstroZombie]

I have 35's no lift and my MPG is GOD AWFUL! 14-15 MPG at best. I see video after video of people getting 18+. What am i doing wrong? HAHAHA. I micromanage my driving too. I didn't expect amazing MPG when i got it but 14 is shit.

I will say i got 19 MPG once driving through LA, of all places. I can get an increase of about 1 mpg if i am watching my shifts and trying to keep it in a gear that cruises under 2k RPM.

I think i'll work on adding 4:88 regrear.

 

[Zachanadandy]

I have 35's no lift and my MPG is GOD AWFUL! 14-15 MPG at best. I see video after video of people getting 18+. What am i doing wrong? HAHAHA. I micromanage my driving too. I didn't expect amazing MPG when i got it but 14 is shit.

I will say i got 19 MPG once driving through LA, of all places. I can get an increase of about 1 mpg if i am watching my shifts and trying to keep it in a gear that cruises under 2k RPM.

I think i'll work on adding 4:88 regrear.

Every time someone documents this great mileage by posting their trip computer I do the math. Typically those nearing 20mpg have an average speed of ~45mph. On the freeway my cruise control is set at 80-85mph. I know why I get 14mpg, but time is money.

 

[Stan H]

I have 35's no lift and my MPG is GOD AWFUL! 14-15 MPG at best. I see video after video of people getting 18+. What am i doing wrong? HAHAHA. I micromanage my driving too. I didn't expect amazing MPG when i got it but 14 is shit.

I will say i got 19 MPG once driving through LA, of all places. I can get an increase of about 1 mpg if i am watching my shifts and trying to keep it in a gear that cruises under 2k RPM.

I think i'll work on adding 4:88 regrear.

Okay I see your running a Rubicon and 4.10's.. well I am too. And I always getting 17-18mph and can get 19mph driving easy. I am running an Automatic. Now when I went to 35's I first got horrible mileage 13.5-15mpg then I bought a Flash-Cal + (plus. ) and recalibrated the tire size in the computer and Bam mileage jumped right up. Took about 1700miles before all the shift settled down solidly but it does settle in as it relearns.

 

[AstroZombie]

Okay I see your running a Rubicon and 4.10's.. well I am too. And I always getting 17-18mph and can get 19mph driving easy. I am running an Automatic. Now when I went to 35's I first got horrible mileage 13.5-15mpg then I bought a Flash-Cal + (plus. ) and recalibrated the tire size in the computer and Bam mileage jumped right up. Took about 1700miles before all the shift settled down solidly but it does settle in as it relearns.

I did that 2 years ago lol and I use Jscan

 

[Stan H]

I did that 2 years ago lol and I use Jscan

Well not sure how the JScan works .. do ypu enter the tire size manually ?
The glash cal + has the tire wizard
You start the wizard and drive at steady speed and it tells you when it's done. And it selects your tire size. I am wearing 35's and it selected 33.25 as my tire size and it's dead nuts on accurate with speedometer etc..

 

[AstroZombie]

Well not sure how the JScan works .. do ypu enter the tire size manually ?
The glash cal + has the tire wizard
You start the wizard and drive at steady speed and it tells you when it's done. And it selects your tire size. I am wearing 35's and it selected 33.25 as my tire size and it's dead nuts on accurate with speedometer etc..

JScan is pretty cool. You manually input the tire size. I will have to check it again. All in with parts was like $75. Full access to the VIN you pay $10 for. It's what teh dealership uses, So i've been told.

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