Should I regear?

[Pescatoral Pursuit]

This is one of those issues where if you have to ask, the answer is "yes."

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[Jeepasaurus_Rex]

I think you should do whatever makes the most sense for you, your driving habits, and your budget. Obviously... this is a pretty obvious answer and probably seems half-a**, but it IS true.

Based on the data you provided, If it were me, I would re-gear.
I'm still toying around the idea of re-gearing mine since I'm on 40's and have a lot of extra weight, but I don't really have MPG issues or transmission/shifting issues... so that's why I'm undecided still.

In my Liberty, it was a no-brainer. I threw in 4.56's because the dang 3.7 motor sucks haha

 

[cutter1138]

Why not use the "manual mode" of the auto trans to keep it in 8th or whatever you want on those long drives?

I have tried that. It does not really help much, at least that what I've found. I think this is a drag problem more than anything else. I have been wondering if the added mechanical advantage of gears would help unload the engine some, additionally around 2000 RPM the torque curve on the 3.6 is very steep so a couple hundred RPM makes a significant increase in torque which would help to hold 8th gear. I don't know if that would change anything on the range though.

 

[redriderjf87]

I feel like you almost can't go too short gearing for the 3.6.

There is enough drag and weight with the JT that you're better off with some extra rpms with a lighter pedal, than stepping in it all the time at a lower rpm. Same mpg and more enjoyable in my experience.

Plus the lower gears are only going to help mpg in city driving anyways.

 

[RavensEyeOffroad]

Hello all,

So, I have been having issues with the way my gearbox is acting for some time, as well as what i would consider dismal fuel economy. Now before anyone says anything, I understand it's a big heavy brick being pushed through the air and that it's not prius (thank God.) But I have been routinely getting 14.5 on the highway.

So my setup is as follows:
JT rubicon with 4.10 gears
2 inch mopar factory lift
35x10.50R17 kenda Klever tires (an experiment that has gone pretty well so far)
ARB Bondi delux full width bumper with 9500lb winch
Best top soft cap for the bed
xtrusion overland XTR-1 bed rack with a small roam case on top

The gearbox holds 7th almost all the time, and will periodically try to go to 8th and then drop down to 7th almost immediately. I have been to the dealer and they were less than helpful. The gearbox has no error codes and the y just put it back into learn mode. I have calibrated the tires on two separate occasions despite getting basically the exact same measurements.

My question is would it be worth regearing? and if so, what gear set would you all use? I have been thinking about 4.88. I want to be able to tow a trailer out west some in the future this requires pulling up some significant mountains. I have gone into the weeds a little bit and have been thinking that a shorter gear would help given the weight and drag produced by the jeep, this would raise the engine RPM some but would off load the engine with the mechanical advantage from a shorter final drive ratio. I would use 8th gear more and might actually see an improvement in range (Mostly what I am interested in for going on long trips).

Am I thinking about this correctly or am I missing something?

Thanks

what I found with reprogramming the computer to the new tire size is that it extends the shift points / rpms. so if before you could get into 8th at 55 mph at 1700 rpms (just throwing out examples, no exact numbers). Now it wants 8th at 58 mph at 1900 otherwise 7th gear 55 mph 1700. basically it has to push the throttle into "downshift to 7th" territory at the given speed/rpm OR it could give light throttle at 7th and hold 55 mph. Drive slightly faster to increase rpms to make it easier to hold 8th. :D

 

[TheRealGinjaNinja]

I feel like you almost can't go too short gearing for the 3.6.

There is enough drag and weight with the JT that you're better off with some extra rpms with a lighter pedal, than stepping in it all the time at a lower rpm. Same mpg and more enjoyable in my experience.

Plus the lower gears are only going to help mpg in city driving anyways.

I share the same sentiment…don’t fear the gear. My Mojave is in the shop getting 5.38s installed and I am running 37” tires. I would highly recommend 5.13s if the OP is sticking with 35s for a while/permanently and potentially going with 37s down the road as it is a great gear for 37s too.

cheers

 

[jensjer]

I have tried that. It does not really help much, at least that what I've found. I think this is a drag problem more than anything else. I have been wondering if the added mechanical advantage of gears would help unload the engine some, additionally around 2000 RPM the torque curve on the 3.6 is very steep so a couple hundred RPM makes a significant increase in torque which would help to hold 8th gear. I don't know if that would change anything on the range though.

Solved! don't use 8th gear.. ; ) 7 gears is plenty, or drive 75-80mph..
7 gears was unheard of 5 years ago in a truck.. it's gonna pick the most efficient ratio for fuel and torque needs automatically. or Sounds like your cruising speed is right in-between 7th & 8th?

Also its real easy to stress those tiny lil pinion gears when going as high as 5.13's, with big tires and lots of rolling mass .. everyone wants top pop wheelies off the line because it's so snappy and fun.. but just adds tons of stress and wear on an even smaller mesh pattern than 4:10's .. People are gonna poo poo this..post but once you change gears it just becomes another maintenance item plan on spending 2-3k every 6-8 years on gears.

 

[Wheelin98TJ]

Solved! don't use 8th gear.. ; ) 7 gears is plenty, or drive 75-80mph..
7 gears was unheard of 5 years ago in a truck.. it's gonna pick the most efficient ratio for fuel and torque needs automatically. or Sounds like your cruising speed is right in-between 7th & 8th?

Also its real easy to stress those tiny lil pinion gears when going as high as 5.13's, with big tires and lots of rolling mass .. everyone wants top pop wheelies off the line because it's so snappy and fun.. but just adds tons of stress and wear on an even smaller mesh pattern than 4:10's .. People are gonna poo poo this..post but once you change gears it just becomes another maintenance item plan on spending 2-3k every 6-8 years on gears.

Do you have data on the tooth surface contact area for the various Advantek D44 ratios?

And what’s the the $2-3k maintenance on regeared axles every 6-8 years?

 

[jensjer]

Do you have data on the tooth surface contact area for the various Advantek D44 ratios?

And what’s the the $2-3k maintenance on regeared axles every 6-8 years?

No data. Just history with D44's
Maintenance = A whole new set of gears, at least in the rear end. gears isn't a last the "life of a vehicle thing" it's mostly the pinion bearing going early taking out everything else. It's the street driving that eats them up (cause you got tons of torque to play with and big tires it's easy to wear them out). Plenty of off road race trucks running ford 9's 6-7.00 gears in short course but you rebuild those every season..

 

[dcmdon]

I've been toying with manually keeping mine in 8th when I'm not in a hurry and cruising along. Typically, the first sign of the slightest incline and it wants to go to 7.
Holding 8 seems to make a substantial difference in the instant mpg and the overall.

I've been doing this for a while. The problem is that with my 35s the engine is turning even lower RPM at 70 mph (about 2000) and it does not have enough power to maintain 70 mph with even the slightest hill.

So even if I hold it in 8th, if I hit any kind of incline on the highway, I have to manually drop it to 7th if I want to maintain speed.

 

[jensjer]

The truck will drive better. But to think you are going to get any kind of real improvement in fuel economy with lower gears is kind of fantasy. The transmission is already compensating by running in 7th most of the time.

All of your mods will work against getting decent fuel economy.

I have a Mojave (same gearing) with a cap and 35s. Stock plastic bumper. At 70 mph on the highway, I'm in the 17s in the summer and about 15 in the winter.

Pushing highway cruising speed up to 75 mph would surely knock another 1 or 2 mpg off.

Drive slower.

Do the math. Unless you are running late for something it's not worth going faster than 70.

I don't understand the lower gas milage in winter.? I always thought the cold air increases with HP.. but maybe it does and that's why it's lower ; )

 

[Mac]

I have a similar setup, mine does the same thing with 7th and 8th gear, drivability and towing are not significantly changed from stock.
Doesn't matter to me when it drops to 7th or 6th gear on the highway, I doubt it is causing any issue either.

 

[Wheelin98TJ]

No data. Just history with D44's
Maintenance = A whole new set of gears, at least in the rear end. gears isn't a last the "life of a vehicle thing" it's mostly the pinion bearing going early taking out everything else. It's the street driving that eats them up (cause you got tons of torque to play with and big tires it's easy to wear them out). Plenty of off road race trucks running ford 9's 6-7.00 gears in short course but you rebuild those every season..

It's hard to find data on tooth contact area. At one time the guys at True Hi9 had data on the 9" and D60 on their website, but it's no longer there. They found the D60 5.13 ratio was stronger and had more contact area than the D60 4.88 ratio. Obviously it's not a D44, but if this is true for a D60 for these particular ratios, it could also be true for certain deep vs high D44 ratios.

Usually there is noise when a pinion bearing is going. If you ignore maintaining bearings, I wouldn't call replacing the gear set maintenance. That is a result of a lack of maintenance.

 

[Wheelin98TJ]

I don't understand the lower gas milage in winter.? I always thought the cold air increases with HP.. but maybe it does and that's why it's lower ; )

Winter blend fuel gives you less MPG.

 

[dcmdon]

I don't understand the lower gas milage in winter.? I always thought the cold air increases with HP.. but maybe it does and that's why it's lower ; )

Cold air does increase horsepower.

Cold air is denser. So there is more mass of oxygen (more oxygen) in a given volume of air. So if your throttle is open a certain amount, in the winter it will let more air in and can make more hp.

But in order to make more hp, your fuel injection meters in more fuel. So for a given throttle opening, you get more air AND more fuel. More power. With more fuel burn.

But that doesn't actually work against you because you need X hp to maintain a certain speed. So you compensate by opening the throttle less.

In the end the throttle is open less to make the same hp when it's cold. I hope this makes sense.

So again, from an engine efficiency perspective cold air is irrelevant.

However what kills you is the actually thickness of the air that the engine needs to push your truck through. Think of air as a fluid. At higher temps it's easier to push your truck through that fluid because its less viscous. That means that at highway speeds, where most of you fuel burn is made up overcoming aerodynamic drag, you will burn less fuel when its warm.

I never realized how significant this thickening of the air in cold weather was until I started flying. The wings on an airplane make lift by being moved through the air. At lower temps a plane can fly at a lower speed because the air is thicker.

I'm looking at the takeoff performance chart of an airplane I used to fly.

At 32 deg the take off roll is 795 ft.
At 85 deg the take off roll is 1010 ft.

Drag and lift are both linearly impacted by the viscosity of the air. So 32 deg air 27% thicker than 85 deg air. (1010/795 = 1.27)

So in short, at 32 deg, the aerodynamic drag your jeep has to overcome is 27% more than it is at 85 deg.

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