2 Wheel mode vs 4 Auto and mpg

[njt07]

Ive had my truck just over a week now and am trying to figure out if driving in 4 auto has a higher impact on normal driving MPG vs 2 wheel drive mode. I assume yes, so have been driving in 2 wheel drive mode but wanted to get others’ input.

I’m about halfway through my 2nd tank and I’m at about 15 mpg which is about what I expect from 75 mph highway and mixed city driving.

I’m gonna drive this entire next tank on 4 auto and see if that has a huge hit or not on mpg, unless of course someone has already done it and tells me.

 

[arod412]

Congrats. I what did you get?

With that said, remember 4wd auto is basically awd. So it only engages 4wd where there is slippage, but for the most part its on 2wd.

Owner of a 2020 Ram Laramie, 16 Dodge Charger scat pack and my wife 21 Jeep Wrangler Unlimited Sahara

 

[jsteinm1]

I'm curious to the impact as well.

 

[mikeru82]

I haven't seen any difference. As stated above, it will be in 2wd mode until you lose traction, and will go back into 2wd mode once traction is regained. So unless you are losing traction a lot I wouldn't expect to see any difference in fuel economy.

 

[njt07]

Congrats. I what did you get?

With that said, remember 4wd auto is basically awd. So it only engages 4wd where there is slippage, but for the most part its on 2wd.

Owner of a 2020 Ram Laramie, 16 Dodge Charger scat pack and my wife 21 Jeep Wrangler Unlimited Sahara

Its in my sig, but its a 3.21 Hemi Limited. TBH I kinda expected a bit better MPG, but Its still in the breaking in stage and I’m being a bit heavy on the gas cause its fun to drive

 

[piening2150]

In my very unscientific 'testing' I see about a 1-2 MPG drop when I use 4 Auto. YMMV

 

[SpeedyV]

You’re spinning a few extra parts in Auto 4WD, even when only two wheels are engaged, so you should expect some parasitic losses.

 

[arod412]

Awesome. The bigger question is that if your going to lift it later, you can throw those numbers away.



Owner of a 2020 Ram Laramie, 16 Dodge Charger scat pack and my wife 21 Jeep Wrangler Unlimited Sahara

 

[dandar]

I noticed about .5 mpg difference. Bigger issue is slight vibration through the steering wheel from the front drivetrain. It's totally smooth in 2WD so I only use 4-Auto when it rains or snows.

 

[njt07]

Yeah I noticed the same. Turned it on in heavy rain last week and I felt that same shudder around corners.

 

[SpeedyV]

Yeah I noticed the same. Turned it on in heavy rain last week and I felt that same shudder around corners.

That’s expected/correct operation, especially in turns.

 

[brian42]

Regardless of what wheels the truck applying power to 4WD Auto locks the font hubs. The front axles will always be spinning so there's going to be some MPG loss with the added drag of the axles whether they are driving or not.

 

[mikeru82]

You’re spinning a few extra parts in Auto 4WD, even when only two wheels are engaged, so you should expect some parasitic losses.

What extra parts are spinning in auto 4wd that aren't spinning in 2wd? Not trying to be an ***, just genuinely would like to know. My understanding of the system is that 4wd auto is controlled by clutches which are engaged by solenoids. And these clutches are spinning whether you're in 2wd or 4wd auto. You are not actively driving the front diff when in 2wd or 4wd auto when there is no slippage. The front diff is always spinning because the hubs are never disengaged. Am I wrong in that assumption?

 

[DOCTORD23]

I have a steep gravel driveway at home, so I put it in 4WD auto when I pull in so I don't tear up the driveway.
I took a 5 hour drive on the interstate to Oregon ( same trip I make regularly) and noticed that I was still in 4WD Auto about a half hour from the destination. I was wondering why my fuel mileage was below normal. Probably as said around 1-2 mpg lower on an eco-diesel.
I'm a lot more observant of it now.

 

[SpeedyV]

What extra parts are spinning in auto 4wd that aren't spinning in 2wd? Not trying to be an ***, just genuinely would like to know. My understanding of the system is that 4wd auto is controlled by clutches which are engaged by solenoids. And these clutches are spinning whether you're in 2wd or 4wd auto. You are not actively driving the front diff when in 2wd or 4wd auto when there is no slippage. The front diff is always spinning because the hubs are never disengaged. Am I wrong in that assumption?

The Ram 1500 uses a "center axle disconnect" (CAD) system, as opposed to locking hubs.

Paraphrasing someone smarter than me: This means that in any 4WD mode (including Auto 4WD), the electronic actuator in the front axle engages, tying the left and right axle shafts together. This starts the front ring and pinion spinning and causes the front driveshaft to start spinning, all of which causes a drop in fuel economy to keep those parts spinning. This is true even if the transfer case has not engaged the clutch plates for the front axle because it has not detected slippage of the rear wheels.

Also see this post for a (very) detailed breakdown of how the transfer case engages on these trucks.

 

[njt07]

Ok so stay in 2 wheel mode on the road, appreciate the info!

 

[mikeru82]

The Ram 1500 uses a "center axle disconnect" (CAD) system, as opposed to locking hubs.

Paraphrasing someone smarter than me: This means that in any 4WD mode (including Auto 4WD), the electronic actuator in the front axle engages, tying the left and right axle shafts together. This starts the front ring and pinion spinning and causes the front driveshaft to start spinning, all of which causes a drop in fuel economy to keep those parts spinning. This is true even if the transfer case has not engaged the clutch plates for the front axle because it has not detected slippage of the rear wheels.

Also see this post for a (very) detailed breakdown of how the transfer case engages on these trucks.

Thanks for the link. It was interesting to read. But it doesn't address the difference between 2wd and 4wd auto, when no loss of traction is occurring. Again, my understanding is that what you called CAD is what sends power to the front diff when there is a loss of rear traction. But otherwise is sending no power to the front wheels. Sut since the front hubs are always locked, the front axles, front diff, and front driveline are always spinning if the wheels are turning. So I would like to know how there can be any difference in fuel economy between auto 4wd (when there is full traction) and 2wd. All the same components are spinning. Or am I missing something?

 

[SpeedyV]

Thanks for the link. It was interesting to read. But it doesn't address the difference between 2wd and 4wd auto, when no loss of traction is occurring. Again, my understanding is that what you called CAD is what sends power to the front diff when there is a loss of rear traction. But otherwise is sending no power to the front wheels. Sut since the front hubs are always locked, the front axles, front diff, and front driveline are always spinning if the wheels are turning. So I would like to know how there can be any difference in fuel economy between auto 4wd (when there is full traction) and 2wd. All the same components are spinning. Or am I missing something?

No, the disconnect happens at the front diff. So when you're in Auto 4WD, it is connected and everything is spinning up front (ready for transfer case to engage clutch plates). But when you're in 2WD, my understanding is the CAD is disconnected and only the outer front axle shafts are moving.

 

[njt07]

Ok so i stayed in 4 auto for the day but had to change back to 2 wheel mode. May have been purely my imagination, but the truck felt different while driving, less smooth... also my mpg dropped down to 14... but that was right after filling up so I should probably have kept it in 4 auto longer to see if it would even out.

 

[Jtr]

Ok so i stayed in 4 auto for the day but had to change back to 2 wheel mode. May have been purely my imagination, but the truck felt different while driving, less smooth... also my mpg dropped down to 14... but that was right after filling up so I should probably have kept it in 4 auto longer to see if it would even out.

I notice a slight "droning" sound/feel at lower speeds in 4 auto so I keep it to 2wd unless the weather dictates otherwise,