I've got factory skid plates as well and I'm sure they clean up aero under there.
Mileage also depends on the 4x4 system used. As THC pointed out a solid front axle tends to hang out there more than an IRS frontend would. Also, most 4x4 have the option of disconnecting the front axle from the wheels but systems like mine have no hubs so the front axle is always connected to the wheels. Nice for on-the-fly but a mileage sucker.
Generally 4x4s arent lowered either so finding a comparison between a stock and lowered 4x4 will be difficult. The only real way I see this working is for you to lower your truck and see what you get , but do it as cheaply and safely as possible. That way if there isnt much of a difference you can always swap back with no problems and not spending any more money.
On the truck forum I frequent, where there are lots of lowered trucks (and even some lowered 4x4s), fuel economy is almost taboo. As soon as anyone mentions it, he gets a flurry of replies saying "If you want to save gas get a Honda".
Occasionally a fuel economy discussion gets missed by the haters. Those threads tend to sound like an HHO thread made by a new user here...people claim a 30% increase in fuel economy from a new muffler, etc. NOBODY on the truck forum ever indicates that they've used a gaslog or accurate pumping habits, and half the time they speak of miles per tank, or just say that they think it's that much better but haven't actually paid attention.
I would guess that lowering in a reasonable way would not pay for itself in fuel economy even over a whole lot of miles...except for a few cheap lowering tricks:
Removing blocks that hold the leaf springs above the axle(s).
De-cranking torsion bars, which I doubt the F350 has (it should have front leafs or coils).
As theclencher says, clamps. There are clamps made specifically for coil springs; I bought them at Pep Boys back in 1997. You could flip them inside-out to lift (by pushing coils apart) or lower (by clamping coils together).
Do check an F350 forum to make sure there aren't side-effects like bad U-joint angles or bottoming-out shocks before trying those ideas.
Found a picture of the coil spring clamp/expander:
In that picture, the assembled unit is setup for lowering. Flip the brackets over and it would lift. I ran one per spring for at least 30,000 miles on a 4,000 pound Cadillac without a problem, just a little clanking if the spring compressed more than they already had it compressed, but a 6,000 pound truck might need two per spring (both compressing the same two coils to eachother) to avoid breaking them.
With regard to the Ford Tempo stock vs. lowered FE: They are gasoline cars, and aero mods don't help FE much because gas engine efficiency gets worse (due to throttling losses) as the load is decreased.
My F350 is a diesel, and the efficiency shouldn't suffer as much as the aero drag is reduced. Still, it's hard to spend lowereing the truck without some guarantee of FE gains.
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