Your not going to see a huge gain dropping down in tire size, a little but not much. If you want to up your mpg take out the lift kit and put on a set of 265/75r16 put it all back to stock height wise and tire wise. Cai is a waste of cash get a K&N drop in air filter.

 

Just a few thoughts here. If you haven't adjusted your speedometer for your tire size then your mileage calculations will probably be lower than actual. Also, if you haven't changed out your O2 sensors you might want to do that and check to ensure your catalytic converter isn't plugged up from a bad plenum.

 

What bjpomeroy just said, speedo calibration. You can use a phone app that will give you gps speed and trip distance so you can get an idea of actual mpg if that's required.

How to change your speedo: Calculate how many revolutions per mile your tire spins then go to someone ( a shop most likely) and tell them you need the "Pinion Factor" changed to xxx (which is the number of revs per mile) for about 40 bucks that should do it, they can also in that same half hour check your O2 sensor switching speed and let you know if everything is ok or not.

For the record, according to my fuel log app I have a lifetime (since I got the app) of 12.7 mpg and yes, I am very tall and running large tires and that is mixed driving, I have a new trans, sure cure kit, shift kit, gm solenoid, deep pan, billet accumulator, 3rd gear lock op, scangauge II which is invaluable in training your brain to manage your right foot...no kidding worth it's money though it really isn't doing anything special. Other things including running mid grade instead of regular to get away from the ****ty blending at some stations. If you have a external transmission cooler you may want to bypass it back to the radiator only since it's not helping in the winter time, I run mine aux first, rad second just to make sure fluid temp doesn't drop below 100 on it's way back to the trans. I also block that half of the radiator.

Run thinner, synthetic gear oil in the diffs, change your transfer case fluid out for DEX VI and wrap your y pipe. I noticed my o2 sensors will cycle the heater circuit over and over again on the highway when in the summer they don't, it also slowed down switching, the wrap pretty much killed that. Might be technical but I also have access to a drb3.

One final thought, jack your truck up all the way around and see how easily it free spins the wheels in neutral. Dragging brakes and even a bad u joint, bearings and so on will all contribute to terrible mileage. Don't forget that when and if you have an alignment done your axle offset is checked, it makes it worse to have the truck dog tracking through rain and snow.