I NEED TO GET BETTER GAS MILEAGE. IT HAS A 6.0 LITER WITH 4.10 GEARS  

TRY SYNTHETIC LUBES IN DIFFS,TRANNY,ENGINE...SHOULD FREE UP A LITTLE BIT...EASIER ON MOVING PARTS,AIR INTAKE,EXHAUST,AND POSSIBLY A GEARVENDORS UNDER/OVER DRIVE.  

Intake is not going to help MPG as mixture is ECM controlled, it is a waste of money. Exhaust will also tend to hurt not help city and urban MPG as well. Gear vendors OD is a waste of money too as you already have OD and lugging engine further will not help MPG. A few things that can help is first, how you drive, go easy on it and keep RPM down (below 3000 when possible will accelerating). Next use smooth treaded tires at or near max rated press as it will reduce rolling resistance. (stay away for oversized and cleated tires as they increase rolling resistance) Change oil often (ever 3K or so) because microscopic grit builds up in oil and it fractionally increases engine drag. Change axle lube and Tcase fluid every 20K or so and for first time after truck has a few K miles on it when new to get break-in grit out of it. Last, stop using 87 octane. This one is kinda important if you are serious about MPG. That engine will never knock on 87 because there is a sensor under intake that listens for the tell tail signs of knock BEFORE you hear it and retards spark to control it. The "problem" with this is that it reduces efficiency too when doing this. The compression ratio in that engine is over 9.5 to one and you CANNOT run 87 in that CR without timing trickery. (the only reason that there is a knock sensor is to limit consumer complaints with cheap gas, not for emissions). This knock sensor keeps you in bliss with no knocks while stealing your MPG. Silly part of this too is that at todays fuel prices, if you are averaging say 15 MPG now, you only need to get about a .5 MPG increase or more with 89 (or 1 MPG increase or more with 92/93) to pay for or save money using better fuel. You will also find that it will run stronger on it too. Again if max MPG is your goal, DO NOT chop up intake and exhaust as it is well tuned from factory.  

what size tires you have on your waggin,245s, 265s? the taller factory tire can lower the rpm a littlebit.keep that big dog waxed,any thing to reduce wind resistance.base line your mileage to and from work till the first oil change,then try a K/N filter in the air box mine paid for itself in a matter of weeks. ran midgrade cause of better power,hence mileage,jacked the pressure to 65-70 lbs in my ld range Es,used 5/30 weight in the sump,i drive like granpa so not to much strain on the motor ta worry bout.its a big heavy truck but itlle last ya!happy wheelin  

Sno, I respect you so much man but I've had experience with some of the after market add-ons working.  I just recently had to sell my 2001 Chevy 2500 HD because we needed a family vehicle. I almost cried, but got through it cause I traded for a damn nice 2000 Yukon. Anyway, in my opinion the big mess under the hood is nothing more than a big air factory. It sucks air in and pushes it back out.  If you make it easier to breath in and out it will run more effeciently.  Like if you were to go for a run with a dust mask on or say a knitted mask.  On my truck, I had a "fram airhog" air filter(wouldn't use this again knowing what I know now about oiled filters but the dry-flows weren't out yet), Dynomax mufflers, E3 plugs, and performance wires.  I also had 285/75/16 Toyo Open Country A/T tires on it.  With these add-ons my truck ran so much smoother than the day I bought it.  I didn't take off like a rocket from a stop and took my time getting up to speed so I wasn't making the truck push the bigger rubber.  I ran less RPM and got 20MPG running at 60 MPH on the highway and 15 MPG running 75 MPH on the interstate.  This was my experience and have had serveral friends and family with the same results. O yeah, my truck had the 6.0 and 4.10s.  

except for the gear vendors to give better mileage at higher speeds...ive used intake sys, exhaust,and synthetic oils, and got better mileage out of my 99 sierra and got the best of 21 mpg...otherwise i wouldnt suggest it...oh and one more thing, if you dont have crap in the back of your pup then drive around w/no tailgate on,that helps out a bit too cause you dont have a big hand grabbing wind at the assend of your rig.  

keys here are low rolling resistance and low wind drag. As speed increases so does drag and HP required to to overcome it. Everything that you can do to reduce the HP to power vehicle. usually best possible MPG will be in the 50 to 60MPH range. above that drag increases quickly. Many seem to fixate on a after market OD and this would have merit if you had no OD but this is not the case today. Lower RPM further does not reduce the power to move vehicle down the road so engine has to work harder to produce more same power at lower RPM (which means more torque) and you reach a point that as RPM drops and torque need increases efficiency drops. Also you must remember that OD is not 100% efficient and uses a bit of power itself in the RPM conversion. When you ad a second OD adapter to lower RPM more, you may reduce RPM but even more engine torque is required to deliver same HP and factional power loss in OD is increased due to a added gear set and bearing.