Noob Question: Does 4Hi cause MPG to drop?

[dmwphoto]

I refer back to AMC's study in the 80s........ <1 mpg

Looking back at the EPA estimates for the Eagle over the 1980-1988 timeframe, and considering when they were SelectDrive vs. full time, I see 1 mpg difference on the EPA ratings.

I seriously doubt there is ANY useful correlation in that study in relation to todays vehicles.

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[Jefe1018]

What extra parts are you rotating making it "alot" ?
You engage gears in the transfer case that in turn spin the front drive shaft.
The front drive shaft turns the pinion which turns the ring gear and carrier. That's it.
Your added "rotating parts" are front drive shaft, front pinion, ring gear and carrier.
Those aren't hard to turn and once turning, you have overcome their inertia or resistance to motion. Keeping them turning is not a lot of effort.
The front axles always turn, all the time it's moving. those in turn spin the side or carrier gears and the carrier pinons on the pinion shaft in the differential. So the axles are spinning one way or another. There's a lot of resistance to spinning the gears inside the carrier while in 2 wheel drive because they are small gears and spin fast on a solid shaft. The left axle spins the right axle stub in the opposite direction as you drive in 2 wheel drive so you are always spinning half the right axle.
You certainly aren't doubling the effort - the rear wheels are already helping the front to roll. There simply isn't that much more torque required from that engine to run a few extra parts which are already being propelled.

I refer back to AMC's study in the 80s........ <1 mpg

Looking back at the EPA estimates for the Eagle over the 1980-1988 timeframe, and considering when they were SelectDrive vs. full time, I see 1 mpg difference on the EPA ratings.

I love your insights, but I have to ask because most of the time it’s exhausting trying to read your rebuttals to the most subtle of “inaccuracies”… do you have pretyped answers already or rely on ChatGPT or something?

 

[ShadowsPapa]

I seriously doubt there is ANY useful correlation in that study in relation to todays vehicles.

Why?
Today's vehicles are even more efficient, meaning there should be less difference.
It takes less fuel for a vehicle to move the same weight today.
You can take a vehicle of the same weight and move it farther on a gallon of fuel than you could decades ago. You extract more power/energy from the fuel so can do more work. In that regard, it's likely less of an issue.

But one can always ask - why would it matter- if you need 4H use 4H, be safe in the snow and don't sweat a mpg.
Snow itself is an issue. Try pushing a Jeep in the snow vs. on bare pavement.
Pushing a 5,000+ pound brick through the snow isn't easy! Betcha lose a couple depending on the snow type and depth.
So not sure why it's a question. What's 1 or 2 mph on a brick that isn't winning any EPA contests as it is.

I love your insights, but I have to ask because most of the time it’s exhausting trying to read your rebuttals to the most subtle of “inaccuracies”… do you have pretyped answers already or rely on ChatGPT or something?

Fast typer, and when you know things for years and years......... it just comes out. Years of work on a keyboard in IT, I suppose?
I also went to college for automotive, ran a shop twice, and worked in the field for years. (besides restoring cars and parts today)
I don't mess with any AI.
I leave that to Lunentucker and his fantastic art work.
I hope he does some more.

 

[DrewsJT]

What extra parts are you rotating making it "alot" ?
You engage gears in the transfer case that in turn spin the front drive shaft.
The front drive shaft turns the pinion which turns the ring gear and carrier. That's it.
Your added "rotating parts" are front drive shaft, front pinion, ring gear and carrier.
Those aren't hard to turn and once turning, you have overcome their inertia or resistance to motion. Keeping them turning is not a lot of effort.
The front axles always turn, all the time it's moving. those in turn spin the side or carrier gears and the carrier pinons on the pinion shaft in the differential. So the axles are spinning one way or another. There's a lot of resistance to spinning the gears inside the carrier while in 2 wheel drive because they are small gears and spin fast on a solid shaft. The left axle spins the right axle stub in the opposite direction as you drive in 2 wheel drive so you are always spinning half the right axle.
You certainly aren't doubling the effort - the rear wheels are already helping the front to roll. There simply isn't that much more torque required from that engine to run a few extra parts which are already being propelled.

I refer back to AMC's study in the 80s........ <1 mpg

Looking back at the EPA estimates for the Eagle over the 1980-1988 timeframe, and considering when they were SelectDrive vs. full time, I see 1 mpg difference on the EPA ratings.

But you forget about loads/resistance. Since the front and back rears are tied together in 4wd and each wheel travels in a slightly different arc even in slight turns(ie bends in roads) one axle is esentally pushing(say rear) and one axle is resisting(front) albeit slightly.
If you look at our gladiators, they've changed some things from the jks. 1. The charging system reduces output way more than the jks did when the battery reaches 100%.
2. The oil pump has a bleed off at reps below 2k ish.
Look at your oil pressure gage go from 29-31 while cruising to 70-80 nearing recline. My 17 jk idles at 35ish my jk 29-30

 

[ShadowsPapa]

But you forget about loads/resistance. Since the front and back rears are tied together in 4wd and each wheel travels in a slightly different arc even in slight turns(ie bends in roads) one axle is esentally pushing(say rear) and one axle is resisting(front) albeit slightly.
If you look at our gladiators, they've changed some things from the jks. 1. The charging system reduces output way more than the jks did when the battery reaches 100%.
2. The oil pump has a bleed off at reps below 2k ish.
Look at your oil pressure gage go from 29-31 while cruising to 70-80 nearing recline. My 17 jk idles at 35ish my jk 29-30

Two stage oil pump, dual displacements. 29-30 belove 3,000 rpm, 70-80 over 3,000 RPM. No need for the extra pressure at lower RPM so save the load on the engine.
In cold weather, because batteries take so much more to charge, I never see voltages below 14.
It typically runs 14.8-15.0 in the cold weather (40 and below)
If I do a full charge of each, and drive for an hour, I see lower voltages (and less load on the alternator)

The differences in the drive train as far as how each wheel travels, pretty much nill on the interstates. Turning, exits, ramps, a lot more.
Your wheels will slip and compensate in the snow.
But it's part of the reason for binding in a parking lot when you hit a clear spot. That and the fact that the conventional u-joints in the front axles make the wheels run inconsistent RPMs as you turn. They speed up, slow down, speed up, slow down, that's part of the jerking feel when turning in 4 wheel drive. Sharper turn, more RPM change of the wheels on a turn causing jumping.

 

[DrewsJT]

Two stage oil pump, dual displacements. 29-30 belove 3,000 rpm, 70-80 over 3,000 RPM. No need for the extra pressure at lower RPM so save the load on the engine.
In cold weather, because batteries take so much more to charge, I never see voltages below 14.
It typically runs 14.8-15.0 in the cold weather (40 and below)
If I do a full charge of each, and drive for an hour, I see lower voltages (and less load on the alternator)

The differences in the drive train as far as how each wheel travels, pretty much nill on the interstates. Turning, exits, ramps, a lot more.
Your wheels will slip and compensate in the snow.
But it's part of the reason for binding in a parking lot when you hit a clear spot. That and the fact that the conventional u-joints in the front axles make the wheels run inconsistent RPMs as you turn. They speed up, slow down, speed up, slow down, that's part of the jerking feel when turning in 4 wheel drive. Sharper turn, more RPM change of the wheels on a turn causing jumping.

But the loads are there along with moving parts and bearings all add up.
Clearly there is more friction that's why the change in mileage.

 

[Gvsukids]

Ah thanks fellas. I figured (and knew it was an obvious answer). I do love me some 4Hi during snow covered roads!

2wd is more fun.

 

[Cape taco12]

My MPG drop like a stone anytime there is plowable snow on the road surface. Mostly because I am revlimiter assaulting snowbanks and going sideways any chance I get.

 

[whysoserious]

I seriously doubt there is ANY useful correlation in that study in relation to todays vehicles.

Especially since OP reports a approx. 3 MPH loss... which seems to negate the <1 MPH in the mentioned study.

 

[whysoserious]

This is probably a dumb question - but is it normal for MPG to drop significantly when using 4Hi to drive on snow covered roads?

Normal MPG on with my Sport (850RE) is around 17mpg in the winter with 2WD.

Had some big snow falls/blizzards recently here in WI and have been driving a lot on 4Hi on snow/ice covered roads and my mileage is around 14.

Thanks.

If you haven't gathered by now, regardless of all the technical "stuff", the answer is yes.

 

[whysoserious]

Why?
Today's vehicles are even more efficient, meaning there should be less difference.
It takes less fuel for a vehicle to move the same weight today.
You can take a vehicle of the same weight and move it farther on a gallon of fuel than you could decades ago. You extract more power/energy from the fuel so can do more work. In that regard, it's likely less of an issue.

But one can always ask - why would it matter- if you need 4H use 4H, be safe in the snow and don't sweat a mpg.
Snow itself is an issue. Try pushing a Jeep in the snow vs. on bare pavement.
Pushing a 5,000+ pound brick through the snow isn't easy! Betcha lose a couple depending on the snow type and depth.
So not sure why it's a question. What's 1 or 2 mph on a brick that isn't winning any EPA contests as it is.



Fast typer, and when you know things for years and years......... it just comes out. Years of work on a keyboard in IT, I suppose?
I also went to college for automotive, ran a shop twice, and worked in the field for years. (besides restoring cars and parts today)
I don't mess with any AI.
I leave that to Lunentucker and his fantastic art work.
I hope he does some more.

Fast typer, and when you know things for years and years......... it just comes out. Years of work on a keyboard in IT, I suppose?
I also went to college for automotive, ran a shop twice, and worked in the field for years. (besides restoring cars and parts today)

You forgot about being retired...

 

[ShadowsPapa]

Especially since OP reports a approx. 3 MPH loss... which seems to negate the <1 MPH in the mentioned study.

You forgot about the in snow bit - that slows a vehicle considerably.
We aren't comparing dry pavement and no change in rolling resistance due to road conditions.
Have you pushed a vehicle through snow vs. on a smooth road? It's a lot tougher. I've pulled vehicles out of the snow and it's tough because the snow piles up a bit ahead of the tires offering a constant obstacle is the wheels roll.
You can feel the extra load on the engine going through any snow beyond a trace.
Of course that's a lot worse when the plow is dropped to the ground..........

 

[ShadowsPapa]

Want to kill mpg? Drive around with a 300+ pound snow plow on the front, 65 mph into a 15 mph headwind.
Clue - 5th gear quite a bit, 6th once in a while. 8th? HAHAHAHAHAHAHA

Absolutely love it.

Interesting note that Newton's first law of motion - there was a mistranslation in there, or a misunderstanding of the phrase when it was translated centuries ago.
It wasn't "unless acted upon" but "except insofar" was supposed to mean that there are always forces acting on all bodies and those forces are responsible for any change in a body's state of motion.
“Every change in a body’s state of motion is due to impressed forces.”
Snow is a force acting upon the tires.

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