Power, power, we all want more power. Naturally, most people think of the engine when upgrading their 4x4, since that's where the power comes from. Is your rig a slug in the rocks or on the road? Just bolt on a blower or add some nitrous to your mill and you'll sure have gobs of power. But is that what you really want, or need, for wheeling?
OK, we admit big power is way cool no matter what, even if you can't effectively use it. The problem comes about when the power isn't matched to the terrain, tires, or gear ratios. These factors are different for the type of wheeling you do. For instance, sand or mud running has different requirements than rockcrawling, and regular recreational wheeling is a combination of other needs.
Theory
Regardless of how powerful your engine is, you still need gears to match the relatively small powerband of an internal combustion engine to the surface you're traveling. The transmission, transfer case, and differentials accomplish this reduction. If you had an engine idling at 600 rpm hooked directly to the axle, that baby would be zipping down the road. However, the engine wouldn't have the torque or twisting force needed for acceleration, which is why we depend on gears for the reduction. Remember riding a 10-speed bicycle? You have plenty of power in your legs (engine), but you needed to start out in first gear, then work your way up to 10th as the bike got moving. Your legs are just as strong in either gear, but the distance covered on the ground for each pulse of the leg is more in the top gear, just like the pistons of your 4x4 engine.
Working Together
OK, enough of the basics, we need gears and power. It's the fine line or proper combo that you need and want, but what exactly is best? How do the little four-cylinders and diesels wheel so well when some big V-8s just don't cut it? It's all how the components are matched up. An engine delivers torque, or twisting force, to the rearend and out to the wheels. Torque multiplication is achieved through the transmission, transfer case, and rear axle. With enough gears, even your little legs could move a mountain, albeit slowly, but with enough power you can do away with many of those gears.
The mechanical advantage of gears also makes it possible to match the wheel speed to the powerband of your particular engine, which is why Second gear may be too low, Fourth is too high, but Third is just right. The same goes for tires, as in stock applications with stock gearing. If you increase the tire diameter from 28 to 35 inches, your factory gears aren't low enough to effectively motivate them, and acceleration goes out the door.
Gearing
Let's take an example of an '86 Chevy pickup in bone stock form. The 350 V-8 engine was rated at 160 horses at 3,800 rpm, and 260 lb-ft of torque at 2,800 rpm. A good rule of thumb to figure out your engine's powerband, or most efficient operating rpm range, is to use 60 percent of the horsepower figure as the low point, and 80 percent as the high point. For this engine, this comes out to 2,200 to 3,000 rpm, which equates well with where the max torque figure is. This pickup came from the factory with 3.08 gears and 235/75-R15 tires, about 28 inches tall. Using the formula to figure rpm (mph times gear ratio times 336, then divide it all by tire diameter) we see that at 60 mph, this combo gave about 2,200 rpm, just barely in the powerband, but just right for best fuel economy.
Off road, even the 2.61 low range is barely able to make up for the 3.08 axle gears, providing less than adequate power for off-roading. Now add a set of 33-inch tires, and even at 60 mph the engine is lugging around 1,800 rpm, way out of the powerband of this engine. In this case, more power can do the trick if you can get the torque and horsepower figures to work at 1,800 rpm. The easier way is changing the axle gears, where 3.73s would be a good compromise, as the chart shows that with 33s and 3.73s, the 60-mph highway rpm would be 2,200. Likewise, choose a set of 4.10s for towing, or 4.56 for better wheeling ratios.