Why the fuel economy difference for 4x4 vs 4x2?

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So I was looking at figures on fueleconomy.gov for the 1999 Cherokee Sport 4x4 I purchased last month. The updated rating for my model, a 4.0L with a 5-speed, is 15 city/20 highway. The rating for the 4x2, also with a 4.0L and a 5-speed is 16/22.

So my first thought is that the 4x4 equipment adds extra weight, bringing down the economy average. A quick look reveals that my 4x4 weighs in at 3313 vs. the 4x2 which weighs in at 3194. A difference of just 119 pounds. I find it hard to believe that you'd knock the highway fuel economy down to 20 in the 4x2 by carrying a female passenger or a couple bags in the trunk.

So the next thought is the 4x4 system itself introducing resistance with additional drive line components and bringing down the efficiency. On a full-time 4x4 vehicle I can see this, but on the Cherokee you're driving around in 2HI 99% of the time anyway, so unless the free-wheeling of the front axle or loss in the transfer case is causing the efficiency hit, I'm not sure where it would be coming from.

The only other thought is that axle ratio on the 4x2 is different, but I don't know what the ratio is on the 4x2 to compare them.

Anyone have any thoughts?
 
More weight, Taller = worse aerodynamics, usually wider more aggressive tires = more rolling resistance. More shafts turning through bearings (even if not transmitting power) = more friction.
 
Plus most 4x4's sit higher, which causes an increase in drag.
 
Not to mention, say on a pickup truck, you typically go from a 3.23 or 3.31 final drive ratio to a 3.73, 3.92 or 4.10 final drive with four wheel drive options.

Typically tires are not any wider but "all terrain" tires are much softer than stock tires. Usually stock tires are fairly low rolling resistance and not that aggressive even with four wheel drive because of mileage concerns.

You have a permanent weight penalty and rolling resistance penalty, as well as a minimum of three inch ride height increase.
 
A 99 Cherokee will use a live axle setup. That means regardless of if you have the 4WD engaged, the front differential will be turning when the wheels are. This kills fuel economy.

Also, transfer cases and front axles are heavy. A lot of weight is added.

Lockout hubs helped with fuel economy, but manufacturers got rid of them because people are too lazy to step out and engage them. Auto hubs are notoriously unreliable, so everyone just went to live axles which suck for economy and wear and tear, but they keep lazy people happy. My Explorer uses a bizarre vacuum disconnect. It essentially disconnects the front diff from the axle shafts (or maybe just one axle shaft) when in "2WD" from what I have gathered. May as well be full time 4WD, and I get awful fuel economy anyway.

Decent 4WD systems that are worth anything off road will always kill fuel economy.
 
Yep, the solid front axle will provide a bit of rolling resistance on a 1999 Cherokee Sport 4x4. It will have a front axle disconnect that 'breaks' one side of the front axle in 2wd mode, but even with this setup, half of the open front diff will be churning away any time the vehicle is rolling. Having owned a 2wd Cherokee, a 4x4 model is well worth the 2mpg loss. Even with snows, weight and winter skill, that baby was way too unpredictable for me.

Joel
 
Well, the specs I've seen show all Cherokee models at 63.9"-64" inches tall, regardless of drive system, so at least for this model, that is ruled out.

Tires on my vehicle are the stock size and match that of the 4x2 model.

It's true that the front axle does spin even when not engaged, and I realize that the transfer case, even when in 2HI, is introducing additional parasitic loss, but I don't see it robbing 2 mpg highway.

All of this said, I took the vehicle on a trip up to Long Island about a month ago and got about 23 mpg with it, so I'm certainly not complaining. Just curious where the EPA difference is coming from.

JTK, IMO, the Cherokee is not a good car for those who want something 'safe and easy to drive in the snow'. Perhaps models with the selec-trac full-time 4WD system do well, but unless the road is slippery or snow-covered enough to drive around in 4HI, this car spends most of its time in 2WD-mode. Even then, it's unpredictable and very easy to throw the back end around. If I make a right turn off a light and accelerate even a little too hard, I can get the rear wheels spinning. It's also very easy to lock up the brakes, but I'd imagine that ABS models do a little better in that regard. If I want point-and-shoot safe snow driving, my Monte Carlo with ABS, traction control, and FWD is worlds better. People who buy vehicles like this to feel safe in the winter are quite foolish, IMO.

Where this thing really excels is doing real off-roading, traversing obstaces, and driving across rough terrain.
 
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Yes, as others have said, it's the spinning axles and driveshaft that reduces your economy. Our first Wrangler was the only vehicle that I've ever owned that could actually slow on a down grade. But even my Warn lock out hub kit yielded no substantial fuel economy increase. The only difference was perceived power. I think that the engine just doesn't fuel squeeze too well ..which is sorta borne out in the marginal difference in EPA economy figures. You can easily beat those numbers in a Cherokee on long trip mileage. The 4.0 appears to take a decent penalty for warm up, as brief as it is. My son's 2000 XJ Sport gets around 25 on a long trip. My wife's Wrangler, which is a little heavier, has a hard time breaking the low 20's on extended long trip driving. 25 miles one way commutes return a common 17.5-19.x routine economy.
 
But the " It's a Jeep thing, you wouldn't understand" ..Priceless :) Heh heh, My old Willys Overland do Brasil had a certain penache, that all my various SJ wagons have had in varying degrees. XJ s have it too. The weight distribution of the wagons in general makes them good in snow and sand. The I6 engine in your XJ is about the best little 6 ever made. But Jeeps never had good MPG. It is like driving a shed down the road. The drag coefficent is huge. Add some big tires and/or a lift kit....
 
In tested form there would only need to be a few minor differences to account for the 5-10% difference in economy implied by the EPA figures. Don't forget those numbers are rounded to some degree, the actual difference could have been less than the rounded numbers let on.

I'm sure you can squeeze out more mileage by driving carefully.
 
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