Adjusting your tire pressure is one of the most overlooked and yet most beneficial things you can do to improve your vehicle's ability and increase your enjoyment of back-road travel. Even if you are just zipping up a well maintained forestry road softening your tires will smooth the ride and increase your traction.
The first thing you need to realize is that your tires are part of your suspension. A hard tire will bounce on every little rock and bump and send those vibrations up to your axles, but a soft tire will soak up a lot of the smaller stuff and help save your teeth from rattling.
This picture shows perfect tire pressure. The tire has conformed to the rock, providing traction and control without flexing so much that the wheel is in danger of being damaged. If the tire pressure was higher the "contact patch" between the tire and the rock would be tiny, and the rig probably would not have been able to climb the rock at all. In addition the flexibility of the tire lets it conform around and absorb sharp edges instead of being punctured by them.
There is no one easy answer to what tire pressure to use, but here are some general guidelines to get started with, based on a near-stock SUV with All Terrain tires:
- For a smoother ride and better traction on maintained dirt roads drop to 20 PSI. At this pressure you don't need to worry about losing a tire off the rim, and you can drive short distances at highway speed. Don't drive for long distance on the highway, as the tire will heat up and can get damaged.
- For rough unmaintained roads, tough hill climbs, snow, or anywhere you need a bit more traction then drop to 15 PSI. At this pressure you can still travel at 60 kph without worrying about pulling a tire off a rim, but at any faster speed you need to be very cautious about hard turns. You can still drive on the street, but you shouldn't go on the highway.
- For serious trails, rock crawling, or if you have gotten stuck at higher pressures, drop down to 12 PSI. At this pressure you have to be cautious about hard turns and you shouldn't travel on pavement for any longer than absolutely necessary.
- In very difficult situations you can drop down to 8 PSI. Keep to a crawling speed, be very slow and careful when turning sharp, and re-inflate your tires as soon as possible. At this pressure there is a very real danger of peeling a tire off the wheel, so you should avoid going this low unless you have a full sized spare.
Extreme wheelers with big tires often consider 6 psi to be normal pressure and will go as low as 2 psi using special BeadLock wheels, but I don't advise using pressure that low unless you are prepared to lose a tire off the wheel.