I'm resisting the impulse to say "sell it and buy a Prius" but I actually do get it - when you have to pay for gas, it makes sense to get the best MPG you can.
To answer your question realistically, we need more information. Year? 2wd or 4wd? Miles? Gear Ratio? What other mods do you have?
What kind of driving are you doing? Stop-and-go city or suburb driving? Long highway miles? With a vehicle as heavy as these, stop-and-go will kill your MPG no matter what else you do.
Also how are you calculating your mileage? If you're going by the DIC readout consider that unless you have re-calibrated your speedo and odo that is going to give you an inaccurate rating. Also, the DIC is not really a good way to determine actual MPG anyway, to do that you need to hand calculate.
If you want to accurately measure MPG the first thing you have to do is figure a way to calculate your actual miles driven.
Here's how I did it on my Suburban after I put bigger tires (33's) on: Get a GPS mounted. Then go to the trip meter on the GPS. Set the GPS trip meter to 0, and set the truck trip meter to 0 also.
Drive at least 100 miles with the GPS on (more is better.) Make sure the GPS is always recording mileage driven. After at least 100 miles compare the two. If you have oversized tires, likely the GPS will show more miles than the truck odometer (because your bigger wheels have a greater circumference the odometer only knows how many revolutions your driveshaft is turning, not how many miles you are actually going.)
Compare the two numbers. Let's say the truck odometer shows 350 miles and the GPS shows 360.3. 360.3 / 350 = 1.03. That means that from now on, you have to multiply the truck's odometer reading by 1.03 in order to get an accurate distance.
Set the truck odometer to 0 when you fill the tank. Drive as normal. When it's time to refuel, fill the truck up and write down the number of gallons it took to fill the truck. Multiply the miles x 1.03 (or whatever your multiplier is) to get actual miles driven and divide by gallons used. That is your ACTUAL mileage (as opposed to what's on the DIC.)