Tire Size Effect on MPG

I am wanting to get the best MPG out of my new to me 2007 2500 CC LBZ with 248k on the clock. I drive MOSTLY unloaded, in Florida, and im only seeing 16 -17 mpg. The tires on the truck are very new 285/75/16 , and I do correct for larger tire size in mileage calculations, and use GPS to compare. Im wondering how much the larger tires effect the fuel mileage, and what difference I would see if I went back to the 245/75/16 size that came on the truck. From what I have found, my tires are 2" taller, so it raises the truck 1 inch. I have always heard lower truck = better mileage, but I dont know how much 1 inch will change things. Any experience / suggestions would be greatly appreciated.
I am also planning to install a DSP5 and EFI tunes at some point. I want the economy tune, and the ability to switch to more power when I need it. I think this would give the best results for better mileage, over messing with the tires.

Scooby,
I would recommend using a Kennedy ECM tune. I have several tunes (Predator Diablo
sport & Edge Evolution) that I do not like because of high EGT issues under load. The Kennedy tune gives a nice boost in HP and fuel mileage (25 MPG highway under 65 MPH) with relatively low EGTs. Additionally, I would stay with the 285/75/16 tires or smaller. I know someone that put a lift kit and 35" tires on his Duramax and now when he hits a hill (even a small hill) pulling his 40' toy hauler the truck goes into limp mode because the gear ratio is way off and it forces slipping in the transmission. My truck is a dually and the factory tires were 215/85/16. I converted it to 225/70/19.5 and had Kennedy do a 5% speedometer adjustment to the ECM when he installed his tune. The 19.5 tires have worked out very well on my truck but I would not recommend going to any larger tires on a dually. Good luck with your truck mods.
John