My name is Rick I have an 06 suburban 5.3 that I bought ayear ago. Can't get any better than 16 mpg on highway trips. Others tell me they get 18-19 . Put in new plugs & wires at 100,000 mi. Don't know where to look.

What kind of plugs did you use?

alot of times its the tires, they are one of the most important things on the car. keep them stock size and most times you can put 2 or three more pounds over stock in them without uneven wear.

the biggest thing is the driver. i always gain the most mileage by leaving the dic in either instant or average mileage mode so you can see what the vehicle likes. if you dont have one there are a bunch of ways to get it through the obd port. i have "torque pro" for android and a odb2 to bluetooth adapter 40-50 total investment. it works on all 96+ and gives you anything you want to display

Suburban...big heavy v-8. All you can do as recommended above tires 3lb over and loose interior extra weight if possible...
Best state of tune up. New air filter,plugs, synthetic engine oil & fresh drive line gear oil/ lube etc. Aggressive or wider tires in any fashion and bye bye to about 3 mpg+.
Run 2 bottles in fuel tank next long hwy drive,and you will clean injectors/valves and get better mileage too.

Link to Chevron Techron Concentrate: did not load..hmmm..
Chevron Techron Concentrate Plus, 10 oz: Automotive : Walmart.com

I used A C Delco 41-110, which is what all the auto parts stores showed. However they were not the same as the originals

Thats prolly because it had platiniums in it they changed them to iridiums. Well you used good plugs. Does the suburban have the stock size tires on it and do the people you have talked to have the same size as you? Also the speed you drive can affect your mileage and wether you use cruise or not. You should find out how the others drive ie. speed. cruise or not.

How fast are you going on your highway trips? I've got the 5.3 in my 2000 GMC and there is a big difference in cruising 55, 65, 75. You want to see what kind of mileage you get at different speeds and find the one that works best for your vehicle.

+1 on cruising speed! I was out in my truck on the weekend, drove about 30 miles on 2 lane roads between 55 and 60, gas gauge barely moved. Came back on the freeway cruising 75-80 and then the gas gauge started dropping a lot quicker! Also the gearing can make a reasonable difference.

What about the affect of an open intak? I know you get more horsepower.