There are many automatic tire deflators on the market today. For us, we have found automatic tire deflators from staun to be the most cost effective and practical for most applications. Just screw on and go. Your tires will be at your pre set level when you hit the trail head.
For the people who want the best. ARB offers solutions for on board air compressors. These however require professional installation, but can be the most convenient option available. Just hook up your hose and you are ready to inflate your tires quickly.
Portable AIR compressors are an affordable option to re-inflate your tires after off roading. These can carried with the rest of your gear and taken tire to tire to complete the inflation. Just hook up to a 12v cigarette lighter and you are done.