Looks like the MPG improved as the final overall drive ration became numerically higher? In both the lifted and unlifted case?
Every gear ratio has it's ideal tire size and vice versa. This is determined by what engine/trans your vehicle has and how you drive it. For our trucks, we have a low revving, torquey V8 or big V6 with a final transmission gear of .7-ish depending on exactly what trans you have.
These trucks came with either 3.42 or 3.73 gears, and you could get the 2500s/3500s with 4.10. The general rule of thumb for these trucks (for general purpose daily driving) is:
3.42/3.73 for stock size tires (31"-32")
4.10 for 33"s
4.56 for 35"s
4.88 for 37-38"
5.13+ for anything bigger (At this point you are getting into serious custom stuff and probably have a bunch of other mods that affect how the truck drives)
Now, if you have a specific use for the truck in mind you can go up or down a gear ratio to tailor it to your specific use. Are you a grandpa just cruising around, going to church and going fishing? You would want "taller" gears to keep your cruising RPMs lower on the highway, keep the truck quiet and save some gas. This hypothetical grandpa probably got 3.42 gears and kept his stock all-season tires. Or maybe you plan to use it as a work truck, load it down with racks and toolboxes and tow a trailer around town. You would want "lower" gears like 3.73 or 4.10 (still assuming stock tires) to help get all that weight rolling. This also applies to oversize tires. With 37s, 4.56 gears would have been closer to stock by the numbers, but those tires are wayyyy heavier than stock and I'm pushing a lot more air resistance being lifted so thats why people end up recommending 4.88s.
Taller (lower numerical) gears give you lower cruising RPMs and a higher theoretical top speed but worse acceleration, towing and hill climbing. Lower (higher numerical) gears give you the opposite: stronger acceleration/towing/climbing but higher RPMs on the highway. Too far in either direction kills your MPG and makes the truck drive bad.
Too tall (like me with 3.42s and 33s/37s) and truck STRUGGLES to maintain speed on the highway and is constantly shifting between 3 and 4 to try and find some power to drag itself up the next hill. This is one way that transmissions die early. Ask me how I know.
It also sucks off-road because it's hard to smoothly deliver power. Imagine you are trying to ease yourself up a big rock. You slowly start to push on the gas... nothing... so you push a little more... still nothing... nothing... OOPS too much now you're just spinning and digging holes.
Too low and it's like driving a tractor. Have you ever tried to drive a tractor on the road? You mash the pedal to the floor, the engine is screaming... and you going all of about 12 miles an hour, and getting "gallons per mile".
But the good news is that your power delivery at low speeds, with heavy loads and up hills is smooth and strong.
But... around here it's usually a broke kid with his first truck, can't afford a lift or a gear change... Just trim and throw on some used 33s and cool wheels with your 3.73s.
You can usually get away with being one gear ratio or tire size off and be fine. Any more than that and it'll suck driving and waste gas. Even for me 3.42 and 33s was fine, until I started to tow more, add racks, boxes, steel wheels, etc. Then I started thinking I needed lower gears... then that turned into "If I'm going to pay for gears anyway, I might as well lift it and get even bigger tires... new ball joints and tie rod ends and wheel bearings while I'm in there... then I need a bigger axle and brakes to handle those tires... and lighter wheels to help save weight... and stiffer springs... and might as well get upgrade shocks instead of the cheapo ones the kit comes with... gotta re-do my exhaust so I gotta buy a welder for that..." on and on until all my money was gone!
Keep in mind that a gear change is probably the single most expensive part of the average lifted vehicle. A lift kit is $1000, you can get decent used tires for $100 a pop, wheels for about the same, but a gear change is $2000-$3000 plus the price of a locker/posi if you get one. I sprung for all Yukon parts so mine really was 3k out the door. It was worth it to make the jump from 3.42 to 4.88 but just going up or down one ratio would be a waste.
This stuff isn't cheap, be careful not to dig yourself into a hole you can't afford. I burn $20+ a day in gas just driving to and from work and spent a down payment on a new truck's worth of money to do it. You have to ask yourself: Are you ok with the idea of working the first hour or so of your shift basically for free to cover the cost of gas in your lifted truck?