how about a 2nd gear box like the dual transfer crawlers use, only make it go up in speed instead of down?

As i've said, i've noticed the same mileage with big 33x12.5's as with 30x9.5's because the engine speed is lower. Hills, of course i have to downshift (unless i'm above 2600RPM which means i'm doing 70mph+) and pretty much my gearing puts me at a .70:1 overdrive.
I like having my engine revving lower, even at high speeds, and it takes very little throttle at 2900RPM in 5th gear to keep momentum (i'm doing 110 on the speedo which equates to about 125-130)... and it cruises great. The same speed with stock tires, i'm at 3500RPM and it's definately using the same if not more gas. With lighter 33x9.5's i bet the mileage difference would be in my favour. I will get them for my winter set when my 30's wear out. Maybe it has more to do witht he v6.. which can tolerate lower RPM's compared to the 22re...

Don't semi's have something like a duel t-case. I think they call them "spliters" or something to that effect. Don't they make 5 gears into 10 somehow, they have somekind of knob by the shifer.

Semi's have more than 10 gears, and if you want to see kind of what they use, look up gear vendors over/underdrive units...

I know that semi's have more than 10 gears. I was refering to the over/underdrive unit that I called a spliter. I was saying as an example that a spliter would take 5 gears and turn them into 10. There maybe some unit out there that makes a final drive ratio that suits our friends needs/wants.

ooookkk agreed. and if you read the rest of my post as well, you would have seen me mention gear vendors overdrive... but i dont know if it'd be worth installing it...

thats what I was talking about with the 2nd gear box behind the transfer case. Onlly trucks have that in the differentials. I think that would get expensive to do on our little trucks

basicly, its not worth it. you will probably spend way more than you would save in gas. if you saved gas at all.

i ran 33x9.5's with 4.10 gears for a while, then switched to 4.88 gears. and my gas mileage went from about 16 (compensated for tire size) to about 18. thats with my normal driving pattern, avg 40-55 mph.

on the highway i got about the same with 4.10s as with 4.88s. thats crusing at about 75mph.

Ok, so here is the real answer to this question.

In order to run lower rpms at higher speeds, you have to effectively change your gear ratio, either by taller tires , ring and pinion, or trans or transfer case reduction. If you don't want your truck to be taller, then reduce your axle ratios. You can go from, say, 4:10s to 3:56s, but you will loose bottom end. You can remedy this by using a double down case for off road, but shifting low/high at speed is a bit of a no-no. So the real solution is to change your ratios to achieve lower rpm at higher speeds, and increase engine torque to maintain bottom end. A mild RV cam and header can significantly change your power curve, which would work well with taller gears. A supercharger is also a good option. Turbos are better for higher rpm, superchargers typically make better bottom end torque.

The idea that more power means less gas mileage is a myth. Consider this example. A half ton Chevy truck with an automatic overdrive and a V6 gets less mileage than the same truck with a V8 in mild hills, because the V6 lacks the torque to pull the hills in overdrive, so it runs higher rpm.

I live in Georgia at the base of the Appalachians. There is one particular hill I drive a lot, speed limit is 55. Stock, in order to maintain 55-60, I had to shift down to 4th. After a mild cam and header, it will pull it just fine in 5th.

So change your gears if you don't want to go tall, and add torque. <img src="/forums/images/graemlins/kewl.gif" alt="" />

The problem with changing diff ratios is that you change rpm's all across the speed range for every gear. If you raise the 5th then you only change top end. I think he really just wants the extra leg at highway speeds, not at low speeds. Money wise your never gonna actually save anything by modding unless you keep the truck for 15 years.