Difference between 2H and 4H AUTO on dry roads?

[sunset]

Hello,

If a Jeep Wrangler has both 2H and 4H AUTO and you are driving on dry paved roads, say, in summer, no bad weather to lose traction, what type of difference does it make whether you are in 2H or 4H AUTO?

Does one cause a bit more wear-and-tear on any internal mechanism? Does one affect MPG more than the other? Any operational/maintenance differences, even if subtle?

What makes me ask is partly because the new 2024 Rubicon 392 does not have 2H but does have 4H AUTO and, for the decreased MPG it offers over the 2024 Rubicon X (V6, not 4xe), it made me wonder if the decrease in MPG was entirely due to the 2 extra cylinders or if other factors figured in. For instance, I don't know how much each engine weighs, the V6 versus the V8.

Thanks.

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[AFD]

I'm not aware of any extra wear-and-tear or damage from leaving it in 4WD-auto all the time, but if I remember correctly, the manual stated that you should expect a bit worse in fuel economy versus 2WD.

Not really sure on all the factors relating to the 392's fuel economy, but no 2WD and the added weight of the engine likely plays a role.

Couldn't find mention of the lower 4WD-auto fuel economy in the manual, but it's stated in one of the instructional videos..

 

[Reinen]

On dry paved roads there is pretty much no difference between 2H and 4Auto. 4Auto is 2H, unless the rear wheels lose traction and 4WD engages. However to have 4WD at the ready like that there is a small hit to MPG due to more drivetrain parts being engaged.

I'm pretty sure the reason why there is no 2H on the 392 is because the engine is strong enough to break the rear wheels free. Only the old-timers know what it's like to drive a vehicle like that (hooray '60's muscle cars!). There are good and very, very bad times to spin your rear tires.

So to avoid litigation from people who don't know how to drive a powerful vehicle, there is only 4Auto so the front end engages when the engine breaks the rear tires loose.

 

[sunset]

Not really sure on all the factors relating to the 392's fuel economy, but no 2WD and the added weight of the engine likely plays a role.

The video you included (starting at time point 00:47) states that the fact that the front axle is always engaged in 4H AUTO (but not in 2h), this mode gets slower fuel economy than 2H position.

So that is one more factor besides 2 extra cylinders (and weight of engine).

However to have 4WD at the ready like that there is a small hit to MPG due to more drivetrain parts being engaged.

Front axle always engaged, per the video and you. Hmm... I wonder if it is only a fraction of a mile per gallon.

I'm pretty sure the reason why there is no 2H on the 392 is because the engine is strong enough to break the rear wheels free.

Yeah, makes sense to me.

That should have been one of my questions, though, and simply put:

Why does the 392 not have/need 2H?

just to find out what everybody says. My Camaro is "2H" (RWD only), 5.0L V8 but is only a 305ci and I don't consider it a muscle car of the 60s by far.

Thanks, guys.

 

[Wbino]

Using 4H auto in my V6 I lose around 1 MPG but it's worth it.
I change to 2H every so often just to move parts around.

 

[sunset]

So that is one more factor besides 2 extra cylinders (and weight of engine).

I just again found that the Hemi 392 is supposed to have MDS which will cut power to four cylinders in order to improve MPG, a "20 percent improvement in fuel economy from cylinder deactivation" as explained here:

https://www.motortrend.com/how-to/chrysler-5-7l-hemi-mds-lifters/

So, even with that 20% working for it, the MPG still sucks on the Rubicon 392! LOL.

 

[sunset]

Using 4H auto in my V6 I lose around 1 MPG but it's worth it.

It's worth it on dry roads even? If so, what do you gain, please?

 

[Wbino]

It's worth it on dry roads even? If so, what do you gain, please?

I'm not mechanic but instances where I hit gravel patches or hitting a bump at speed and you're off the road for a millisecond I imagine might be regarded as slippage.
Plus I'm in the North East so winter driving is a no brainer.

 

[Ratbert]

If your primary concern is mileage why don't you observe your mileage before and after on a flat road at constant speed?

 

[Riotfunk]

Some vehicles will engage the front wheels on takeoff to mitigate spin and improve traction.

I would assume the jeep one does this and can see it on dash if they have one of those live power screens. Did this in another vehicle I had. Hence why worse mpg.

 

[rcadden]

If there's no chance of slippage, I keep it in 2H. However, if it's recently rained, expected winter weather, or anything else where I might abruptly want/need 4WD, then I put it into 4HAuto. It's maybe 1-2 MPG different.

Our local offroad park is also pretty light, so I sometimes will just put it in 4HAuto while I'm doing those lighter trails.

 

[Steph1]

I put my t-case in 4H auto in automn and leave it there til spring. Makes for a fantastic winter condition wrangler compared to without that option. Never officially calculated the mpg difference, but do notice approx 1/2-1 mpg difference which could very well also be winter gas effect.

 

[lindaspins]

When I first got my 4xe I left it in 2H. Tested out 4H auto and never went back. If there's a mileage difference I haven't noticed- maybe I'll try 2H on the next road trip, where the brick into the wind scenario is most noticible, driving at highway speeds into a headwind somewhere in Kansas. My best guess is the MPG's might suck slightly less but still be better than our '15 JKUR under the same circumstances.

 

[sunset]

If your primary concern is mileage why don't you observe your mileage before and after on a flat road at constant speed?

I'm just trying to find out as much as I can before buying a Jeep which may be a 392 where I would have no 2H with which to compare to 4H AUTO. Curious, too.

 

[sunset]

When I first got my 4xe I left it in 2H. Tested out 4H auto and never went back. If there's a mileage difference I haven't noticed- maybe I'll try 2H on the next road trip, where the brick into the wind scenario is most noticible, driving at highway speeds into a headwind somewhere in Kansas. My best guess is the MPG's might suck slightly less but still be better than our '15 JKUR under the same circumstances.

A fellow with a 4xe did an 80-mile road test, once in 2H and once in 4H Auto and compared MPG. 2H was 19.6 while 4H AUTO was 23.8 even though the Jeep people said their engineers found there is "really no difference".



The fellow acknowledged the 2H 80-mile trip was hilly while the other one was not. But a 4MPG difference?

He didn't say how he excluded the battery for these tests while, in a subsequent video, he said he depleted the battery to calculate MPH but that video was not a 2H vs 4H AUTO comparison.

As my vehicle won't be 4Xe, I kinda set his test aside.

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