Buy 24"s, and be done with it. Or go with a 22". Look at it like this, a 23" tire almost costs more than a 24". The reason is, how many people have 23"s, answer, not many. Its your truck, what ever you pick, you will wish you went bigger. Trust me, I had 20", then I bought 24"s, then I saw a guy with 26"s on his ex, and was a bit pissed that I didnt do 6's from the start.
Yes, wow, that looks sooooo coooollll! NOT!!!!
Didn't want to "quote" the pics, as that makes the thread very large, but they look absolutly rediculous. Absolute ghetto trash!
16"s, 17"s maybe, as they may improve handling if done right. Anything more then that, especially on an SUV, and especially 24"s/26"s is just foolish. It shows you know nothing about how a vehicles suspension system works. (Yes, wheels/tires are part of the suspension.)
Sidewall height and air pressure, along with tread compund and sidewall construction/stiffness all contribute to both handling and ride quality. The aforementioned variables control and tire/wheel combinations "spring rate". These stupid large wheels and rubber band thin tires have no spring rate. Unless you are driving on Autobahn quality roads (If you live in the US, you're not driving on good enough roads, period!), it will ride like total crap, and will put extreme stress on all other vehicle components, from the vibration and impact harshness of hitting/riding over potholes, ruts, and even minor road imperfections.
As far as claiming any handling improvements, nope, don't think so. Tires with so little "give" will tend to "hook into" any/all "ruts" and bumps in the road, and follow them. It's called tram-lining. Google it, although I may have spelled it wrong. Also, any minor gains in steering precision you MAY have gained, under ideal circumstances, with such large wheels/low-pro tires, is more then offset by the increased center-of-gravity which results from the 6" lift that you needed to clear the tires, not to mention the increased stress/wear on your driveline/suspension/steering components, just from the lift alone; never mind the added factors from the tires/wheels.
As for performance/hp/mpg/etc....: To address the OP's goal of gearing changes for better mpg, this NOT the way to do it. If you need/want to change your final drive ratio, then change your gears.
With wheels/tires this large, there is no performance gain, period! There will be a performance loss, actually. First, they are heavier than a stock wheel/tire combo, and the wheels are one of the worst places to add weight to a vehicle. Out there, it is not just static weight. It is unsprung, rotating weight. Increasing unsprung weight has negative effects on handling. Rotating weight has an "effective" weight far greater than actual, due to centrifigual force. It takes more energy to acclerate that weight from a stop, and, once moving (turning), it takes much more force to stop it. That means, bigger wheels, means you NEED bigger brakes to compensate. (ok, maybe going to 16", or 17" aftermarket aluminum probably won't, as they are typically lighter than a stock 15") As the diameter of the wheel increases, this effect only becomes more pronounced. Yes, I'm sure a bunch of fools are going to jump in and say "but eye's ain't gots no bigger breaks ons my "laced out whip" yo!, and ain't crashed yet." Maybe they're just lucky, or maybe when they cruise the local ghetto for "*****es", they aren't going more then a crawl. Either way, it's a disaster waiting to happen. Bigger wheels = you NEED bigger brakes. $$$$
Oh yeah, as for that gear ratio "change" from the bigger wheel/tires, first, if you want you're computer controlled tranny to live, and shift properely, you'll need a programmer, such as an Xcal3, to program the new tire size in. Even if you do, don't expect the tranny to live as long as it normally would. You're asking it to work much harder, both from the increased rotating weight it needs to accelrate, as well as from reducing the "mechanical advantage" that gearing provides, by having a "taller", or numerically lower, final drive ratio. Any small mpg gain you might see from that "taller" gearing, on steady state highway driving, will be more then offset by the loss of low end torque multiplication, and reduced acceleration capabilites, at lower speeds, from a stop, and around town.
If all you care about is "bling", and whether or not you can impress Will from "Unique Whips", then sure, why not put 26"s, or even 36"s on it. Hell, how about some chome spinners from the "Escalade" monster truck????
On the other hand, if you want a drivable, safe, sane vehicle, with some reasonable improvements in performance, handling, and mpg; then don't go bigger than a 17", or maybe, if you really must, an 18".