Aftermarket:
Go to the local supermarket and buy a dozen eggs.  Throw out 11 of
them, place the remaining one between your right foot and the
accelerator.  There's very little else you can add on besides a tonneau
cover that might have a positive effect on mileage, but I suspect
the difference is quite small for what a cover costs, and would take
a good while to pay off.

Non-aftermarket:
Use the thinnest oil recommended, keep the air filter clean, the
wheels aligned, the tire pressures at the maximum recommended value,
make sure the brakes are properly maintained to prevent drag, and
keep your speed (and acceleration) under control.  Try to minimize
your use of the brakes, accelerate slowly and never accelerate
toward a red light- back off and coast toward it.  You'll be
amazed at how simply driving like an old man will increase mileage.

There really isn't anything that will make such a difference that you'll
get an overall cost savings or ROI within the life of the vehicle.  For
example, let's do the math...

At 12k miles per year, 18 MPG, and $2.65 p/gal = total yearly fuel of
$1767.  With a 10% increase in MPG, you save $160.

I don't think you'll find any $160 or less aftermarket solutions that
provides a 10% increase in MPG.

Or, some other figures to contemplate...

adding a cold air intake system, headers, and high flow exhaust system
will cost about $1000.  With that you may, and I reiterate "may", get a
10% increase...as long as you keep your foot out of it...because you'll
also get a performance gain that may be hard to resist.  To recover the
$1000 in the 1st year you'd need a %130 increase (equal to about 41
MPG).  Or, going back to only the 10% increase, you'd need 6.25 years
just to break even...are you going to keep your 1998 for another 6 years?

Granted, the more miles you drive per year and how often and what you
tow and carry will affect the numbers.

For me, I drive, carry, and tow alot.  I add aftermarket enhancements
for performance, which in a small way increase my MPG.  But, I'm more
concerned that I can get up a hill, merge into traffic, or pull out of
my driveway.

FWIW, be leery when a manufacturer makes an MPG increase claim.  For
example a claim of 3-4 MPG increase means your 18 MPG truck would need
to be 17-23% more efficient...kinda unlikely.  When looking at claims,
if the manufacturer can't provide exact figures for your vehicle, then
percentage increases provide a better measurement than flat MPG number
increases.

This may not have been the answer you were looking for, but hope it
helps anyway.

There isn't much you can do.  When it's time for tires,  getting the right
tires may net a small savings.

Bill M

I've had and have both a 97 and 02 4L 4x4. One auto, one stick. 16 around
town, 23 highway. No matter what I try. Better mileage in the summer. And
there's a big difference between 55 and 70 mph on highway mileage. I take
good care of the vehicles. Mobil 1 for the engine. New air filter at 12k.
Oil and filter at 4K. Not much more you can expect. They run nice.