Not in the grand scheme, maybe a couple hundred pounds. The amount of force needed to roll you vehicle depends on the weight, and the rolling resistance of the tires. Weight is pretty straight forward, and rolling resistance ranges from ~.006 to .016, so a better bet would probably be LRR (low rolling resistance) tires, since you can likely see way more of a gain there. For instance, if your truck weighs 3500lbs, with the likely Crr of .010-.012, going to a LRR tire may net you a bigger gain. So
Ooh a hardbody! I put 100k on one of those myself. Mine was 2wd. I tried a few mods in the name of power and economy.
9.5 to 1 pistons
tailgate up, down, off
a bed topper
I always seemed to get the same mileage. But I didn't have any instrumentation. And the truck was never properly tuned for the pistons. I generally averaged around 24-26mpg 85% highway. Best ever was 27.1mpg. I generally did full fills.
02 Saturn SL
for pics click the link below
if you take the tail gate off, what are you going to do with the stuff you are hauling?
I say shift earlier, you have torque to haul over half a ton, in other words you have a monster under your hood, if you have an empty bed, then don't drive like you are hauling a load.