Well they probably aren't that far apart in weight when you consider I have the brush guard, stereo/radio equipment and a fully loaded tool box and your truck is probably naked.
Trucks are heavy, I just used it to my advantage. Instead of powering up hills and around corners I would just let it coast and let the weight pull it through (until it got below 5 mph of my regular speed). Like I said, there was a lot of fancy pedal work involved. I need to see if I can get it weighed because I've just been going on estimates. I'm guessing 52-5300# for my truck (4800# curb + 150# grill guard + 100# stereo/radio equip + 200plus# tool box...that sucker is heavy when fully loaded like it is) and I've seen numbers for your truck that range from 5200 for 2wd to 5600 for 4wd. But the road I took has a magical effect on mileage.
And 907dave makes a good point. Until you have your tire size dialed in, your mileage is going to be off. It will be better if the number is smaller and worse if the number is larger. If you dont have a GPS or some decently spaced road markers just go to a parking lot with a marker. Mark the tire and the street, roll one full turn and get a tape measure to measure. The measurement will probably be really close if you drove for more than 5 miles to warm the tires up. It would be funny if driving to a parking lot could increase your mileage.