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Old 08-24-2014, 11:53 AM   #1 (permalink)
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4x4 mech drag - transfer cases (esp double), manual vs auto hubs, axles, etc.

Does anyone have any kind of specific figures for the amount of mechanical drag added by a 4x4 system? For those of us that need 4x4 ability i'm curious how to minimize the drag as close to 2wd levels as possible/I don't understand why there should have to be a 10% or more MPG difference...

For instance i've heard that 100% manual hubs on the front are worth about a 1/2 mpg difference. (some have claimed 1mpg but I don't think i've seen any documentation, yet even 1/2mpg adds up especially on a lower mpg vehicle) Apparently because even with the transfer case disconnected the front axle still freewheels and such causing drag. I would be very curious if this is still the case with modern trucks with the endless chase for MPG out there - whether this has improved and if something like swapping on modern hubs to an older pickup would maintain auto hub functionality, or if even modern ones have an MPG penalty for convenience.

I've heard that solid axle front suspensions exhibit worse mpg than independant ones due to the unsprung weight. I'm not sure why this is, increased drag on the front tires over bumps I could understand but highway should be pretty smooth normally?

The transfer cases themselves and range boxes i'm curious how much drag they add. Especially for things like doubler transfer cases if they make a big difference or not. (as I was hoping to look at steeper axles like 3.07's minimum or 2-something if I could get it made, then use a double rangebox to make it more driveable at other times... but if it adds alot of drag it may not work as well) Or certain models vs other models.

I know different axles have different amounts of drag, although I rarely see specific figures, outside of a Ford 9 inch is often the worst and anything by Dana usually the best due to something involving pinion position.

I've no clue how much would be eaten by different driveshaft or U-joint options. :)

I'm trying to figure out how much % of the total drag comes from different parts, seeking to optimize or change things around, and how to minimize the total system drag added by 4wd at all if I can. Maybe the ultimate answer is just adding FWD electric motors for on demand traction though. :)

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Old 08-24-2014, 02:20 PM   #2 (permalink)
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Well every RWD truck to 4x4 truck there is a weight difference of at least 500lb.
On my suburban adding 500lb makes a 0.5 to 1mpg difference in fuel economy.
To me having 4x4 stuff is 500lb less that I can tow.
Then you have all of that additional crap spinning all the time when you don't need it, that is at least another full MPG down the toilet.
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Old 08-24-2014, 04:45 PM   #3 (permalink)
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I'm disappointed that axle manufactures haven't invented a better way to disconnect front hubs from axles. So most trucks have parts spinning despite 4x4 not being needed, wearing them out. Stillsearching, aerodynamic drag can be a large difference between 2wd and 4x4 in 3/4 & 1 ton trucks if these have a solid front axle. Also transfer cases that hang down drag on the air.

Often, 2wd trucks have lower final drive ratios, say 3.55:1 vs 3.73 or 410. Also, they may be equipped with lighter, narrower and less aggressive tires from the factory.
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Old 08-24-2014, 04:57 PM   #4 (permalink)
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This is pretty anecdotal but it's about all I can contribute. My '90 Dodge Dakota 2WD V6 auto with 3.21 rear gears and 8' bed usually got 21-23 MPG on the highway. Curb weight is a little over 3,000 pounds and it had 205 mm wide tires.

I was a little apprehensive when I got my current Dakota because it's a 4WD. It is also a '90 with the V6 and 8' bed. It does however have a manual transmission, which should yield better fuel economy. On the flip side it's a couple inches taller, has 3.55 gears and 235 mm wide tires, and weighs 700 pounds more (curb weight around 3725 lbs.). About 400 pounds of that is the difference from 2 to 4WD and the remainder is due to the heavy-duty payload package.

I only had a chance to run one tank in the 4WD before I tore it apart to fix some things. I was pleased to average 21.5 MPG on a tank of very mixed highway/city/dirt road driving. That was with a very worn timing chain, slightly dragging calipers, and wheels that stick out past the side of the vehicle. I'm sure it'll do better than that when I finish working on it and get a chance to do some highway cruising.

The old EPA ratings showed 21 MPG on the highway for the 2WD auto and 19 MPG highway for the 4WD manual (a 2WD manual is rated at 22 MPG highway). Anyway, I would need to test a truck that's identical to my 4WD minus the 4WD for a real one-on-one comparison, but I guess I was just excited to see over 20 MPG right off the bat. I've never owned a 4WD truck before.

This truck has a center axle disconnect instead of locking hubs so the front differential is still turning (but "open" - the right axle shaft gets split by the disconnect). The front driveshaft and transfer case chain drive are not turning.

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