Noob Question: Does 4Hi cause MPG to drop?

[Sigz]

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.

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[sharpsicle]

Yes. 4wd always has more power lost in transit than 2wd.

 

[kevman65]

Yes.

You're sending power to all 4 wheels, which is doubling your normal power demand.

That along with worse MPG on winter blend fuel, worse MPG due to cold temps, yadda yadda yadda

 

[Volt0]

Maybe a smidge, just from rotational drag of turning more drive lines. The drop that you are seeing is more likely from denser air and/or worse traction. Or higher winds, or not in 8th, or tires dropped pressure b/c it got colder.

 

[Sigz]

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

 

[ShadowsPapa]

Less than 1 mpg.
AMC did a study in the 1980s on this very topic. They went back and forth between full-time 4 wheel drive and selectable on the Eagle. In the end they found the impact to be so small, under 1 mpg, they settled on taking out the FAD and going full time on the Eagle.

Snow causes rolling resistance, winter blend fuel, wind, etc.
If I have to deal with much snow, I see a drop in mpg.

If you have the SelecTrac transfer case and use 4H auto, you won't see any difference because it releases the clutch packs when not needed.

There's also this bit related to 4 wheel drive in snow - are you pulling the front tires through in 4H, or shoving them through by rear drive only? Pushing them builds snow up ahead of the tire, resisting forward movement, in 4H the tires are pulling themselves through, not shoving at the snow as if being pushed by the rear wheels only.

 

[kevman65]

Less than 1 mpg.
AMC did a study in the 1980s on this very topic. They went back and forth between full-time 4 wheel drive and selectable on the Eagle. In the end they found the impact to be so small, under 1 mpg, they settled on taking out the FAD and going full time on the Eagle.

Snow causes rolling resistance, winter blend fuel, wind, etc.
If I have to deal with much snow, I see a drop in mpg.

If you have the SelecTrac transfer case and use 4H auto, you won't see any difference because it releases the clutch packs when not needed.

There's also the bit of 4 wheel drive in snow - are you pulling the front tires through in 4H, or shoving them through by rear drive only?

Oh, the front wheels are supposed to be touching?

 

[ShadowsPapa]

You're sending power to all 4 wheels, which is doubling your normal power demand.

That's not doubling power demand as you are still moving the same weight at the same speed under the same conditions.
The only differences would be frictional losses in the pinion-to-ring gear area. Otherwise, power or torque required is very nearly the same. You can watch this in JSCAN.
I see no difference in my SX4. In fact, the car is really pretty much the same to push in 2 or 4 mode. You are only turning one extra driveshaft and the only extra friction loss is in the front differential ring to pinion area. Otherwise, you are spinning the same exact parts except the front carrier with ring and pinion. The axles still spin in the carrier turning the side and carrier pinion gears.

You are moving the same mass regardless.

 

[ShadowsPapa]

Oh, the front wheels are supposed to be touching?

Huh?

 

[kevman65]

Huh?

You asked if the front wheels were being pulled or shoved.

I asked if the front wheels were supposed to be touching, as in the snow or the ground.....

 

[ShadowsPapa]

You asked if the front wheels were being pulled or shoved.

I asked if the front wheels were supposed to be touching, as in the snow or the ground.....

Ah, your jeep has way too much power. Maybe consider wheelie bars? Turn that turbo down a notch.

 

[Koolcarguy]

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.

Yes your using more power to turn the front diff and more resistance

 

[TheRealStreetcommander]

Snow causes rolling resistance,

Agree, this is likely the largest culprit. 1-1.5(-)mpg from driving the front gear train.

 

[DrewsJT]

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.

Short answer....absolutely. You're rotating alot more parts

 

[ShadowsPapa]

Short answer....absolutely. You're rotating alot more parts

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.

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