This is the question that has plagued four-wheelers for ages. For serious off-roading, it is a definite improvement over fully-aired meats. However, once you are back on the road, aired down tires can be loud, wear unevenly and be right down dangerous. The risk of a tire coming off of it's bead increases with speed and with less internal pressure, the risk multiplies greatly. (Editor's note: tires are assembled at just over 200 degrees Farenheit. Driving even a short distance on an aired-down tire can heat it up very quickly. Once it's at 200 degrees, it can literally dis-assemble itself and come apart!)

Therefore, it is essential to air back up when you are done four-wheeling. There are a number of small air compressors available that work with your rig's existing systems. These systems are usually $200-400 dollars. I admit they aren't nearly as cool as the Hummer's central air inflation/deflation system but definitely not bad either. (Editor's note: Another option is carrying CO2 tanks, which are much quicker).

Aside from the disadvantages, there are a lot of advantages to airing down. If you use a tire such as the Super Swamper Thornbird that uses very large side lugs, it is most effective when aired down to 8 psi or lower. This allows the tire to conform to the terrain. Large heavy tires with large lugs dig into the terrain much better if they can get a larger "footprint" into what they are running on. This is attained easily with a drop in pressure.

Now comes the next big question. How low can I go? This is a big factor that is based on:

A. What terrain are you tackling?
B. Which tires and rims are you using?
C. What size/weight is your rig?

These questions are ordered this way for a reason. The size and weight of your rig is important, but not nearly as important as the type of tire you are using and where you are using it. Because of this, I have listed the basics of where to air to and why below. All of these measurements are for Thornbirds, specifically but you can apply the same ideals to any tire combination.

SNOW: In snow, specifically snow that is packed and wet, you can air down safely to 8 psi. This will allow the footprint to expand and allow a great grip into the slippery terrain. With snow being slippery and dense, the tires will conform and help to compact it. This will also help others following behind you as a packed powder will be easier to traverse than one that is loose.

MUD: A lot of people think airing down in mud is a bad thing. Since tall thin tires tend to do best in mud, big sidewall lugs don't always help too much. This is your call but I wouldn't go below 20 psi here.

SAND: Where wide tires and big engines are king, airing down is essential. In sand, wide, low psi tires help paddle you along easily where tall, full pressure tires will dig you in. With enough engine power, the lugs of your tires will leave a wide non-digging pattern that helps you stay afloat of the sand.

ROCK: This is essential. Huge sidelugs aired down can engulf rocks and let your rig move effortlessly over them. One lug can often catch the sides of a rock and pull you to safety. Some people prefer many small lugs here but I prefer fewer larger lugs aired down to 5-8 psi as they can usually hold you like to other combination and since you are traversing at such slow speeds, your bead should remain intact.

I hope this helps you all in your airing down practices. I'm looking for ideas for my next column so if you have anything, please let me know. I'll be glad to hear your feedback.

(Editor's notes: Many of today's mud-terrain tires, like the BFG, cannot run at the above low pressures, typically. For example, a 31-32" BFG M-T on a 15x8 rim becomes a hazard much below 10-12 pounds. The chance of losing a bead is great below 10 pounds. I usually run my 32's at 12 pounds in most all off-road situations. The ride is nice and soft and the tires conform very nicely to the rocks. Of course, the lower you go, the lower your vehicle becomes, too. Therefore, it's a constant balancing act between low pressure vs. clearance and ride. Only experimentation will tell you what pressure is right for you.)

Higher than recommended pressure can cause rapid tread wear in the center of the tire, harsh ride, poor handling, and possible tire damage (see Figure 2). Uneven pressures from side to side can cause steering pull or lead, uneven braking, quirky handling and torque steer.

Tire and car manufacturers carefully consider many variables when recommending a tire inflation pressure. Safety, tire wear, handling and ride comfort are all affected by inflation pressure. Recommended inflation pressure can usually be found on the driver’s door jam or in the vehicle’s owners manual. Other than a change in tire size, there aren’t many circumstances that would require deviating from the recommended pressure.

An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when encountering potholes or debris in the road, as well as experience irregular tread wear. Higher inflated tires cannot isolate road irregularities as well causing the vehicle to ride harsher and transmit more noise into its interior.

According to the AAA Foundation for Traffic Safety, 51 percent of American drivers check their own tire pressure. Unfortunately, 48 percent of those surveyed consult their tire's sidewall to find the proper inflation pressure.

"The inflation pressure on the sidewall of the tire is the MAXIMUM operating pressure determined by the tire manufacturer," says BFGoodrich (www.bfgoodrichtires.com). "It is not necessarily the correct tire inflation for your vehicle. You should ALWAYS use the inflation recommended by the vehicle manufacturer. You can find this information in your vehicle owner's manual, or posted on the inside edge of the driver's door or other door, or on the inside of the glove box door.