Another angle on airing down your tires uses products that screw onto your valve stem and continuously let air bleed out. The four air deflators from Tera Manufacturing screw together to form a handy key ring, then simply screw them on the valve stem and they depress your core to let the air out. You can check the pressure with the deflators on the stem, and then simply remove them once they reach the desired pressure, as they won't stop themselves.
The Staun Products tire deflators come in a handy little leather pouch and can be screwed onto your valve stems at base camp and they will automatically deflate your tires while driving to the trail. The cool part is that they will stop at a preset pressure. Unfortunately, you will need to first lower a tire to your desired pressure by another method so that you can set the Staun units to stop at that pressure. We have found the Staun units very useful, but we also cut out the instructions and keep them with the deflators since they can be tricky if you don't use them often or want to change the pressure setting. However, if you know that you'll always air down to a certain pressure, simply set them to that amount and lock them in-then every time you hit the trail simply screw them on and drive. Remember to remove them before wheeling, and if they get dirty you should clean them as the dirt can influence their precision.
Another style of automatic deflators is those made by Trailhead Deflators. These come in a zippered carrying case and they can be preset to a determined pressure by tightening an internal setscrew with an included Allen wrench. Plus the kit comes with laminated instructions and a tire gauge.
The Trailhead kit comes in two versions, a 5- to 20-psi version for Jeep and 4x4 owners, and a 15- to 40-psi version for motorhomes. In addition, the kit includes a helpful tire-deflation guide that can help a novice wheeler determine a safe tire pressure for many different 15- and 16-inch wheel-and-tire combos. Simply set the guide to your tire size, cross-reference the vehicle's GVW, and you are given a recommended pressure to air down to.