The best and safest fix/mod would be to install a low restriction exhast
on all of them as this will increase turbo efficency, give you a little
more free power, raise MPG some while lowering EGT which extends engine
life.

As you might have guessed, with fuel costs what they are and
manufacturer claims being what they say to sell product, this question
has been asks alot recently.  Below are some snippets from other threads
to which I've responded.

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 know any of
the chip guys, or anyone else for that matter that claim a 10% increase
in MPG, so I don't think you'll find any $160 or less aftermarket
solution that provides a 10% increase in MPG.

Or, some other figures to contemplate...

adding a cold air intake system, chip tuner, and high flow exhaust
system will cost about $1000 (low end).  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 vehicles
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.

More at issue are: (1) getting better air flow (intake and exhaust) to
help both economy and performance, as well as EG temps and (2) what's
the ROI when you factor in the cost of the tuner and other mods to the
fuel saving cost.  If you don't do alot of driving and/or towing it may
take a long time to realize the ROI.

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.