Originally Posted by grats
Usually it's marked on the side of the tire for proper inflation on the road.
I have 31x10s and run 10pounds off road.
Sorry bud, but you are wrong there. The pressure listed on the side of the tire is the maximum inflation preassure that the tire is designed for. It does nt take into consideration the type and/or weight of the vehicle it is bolted onto. If you run that pressure on a zook, it will ride like a paint shaker.
Start with the pressure listed on the data plate on the side of the dash board. It should say 20 psi (22 for the rear if you are loaded). Some of us have stock rigs and the pressure is pretty close for regular driving on average tires. Some of us have added heavy bumpers, roll cages, gas cans, etc. that would change the pressure put on the tires. Some tire types/brands have heavier sidewalls that change how low you can go.
Start with 20psi.
Drive through water and then then dry road and check your footprint. If the footprint is the full wifth of the tread, and it is clearly the same across the tread pattern then you are right on target. If the print is skinnier than the actual tread width then there is too much air in the tires. If the print mark is lighter in the middle, then you have too little air pressure, so bump it up a little.
There are other, more technical ways to check, but this is the easiest, most basic way.
For offroad, all bets are off as there are many other factors. Less air pressure leaves a larger footprint which equals more traction, less disturbance of the trail (makes the environmentalists happy while getting you farther down the trail!). But if you go too low you can lose a bead or slice a sidewall (most street tires have thin sidewalls). Here are some pressures I have used:
Stret tires - anything that you would put on a normal road sedan
street tire / on road - 20psi
street tire / off road - 15psi (but be carefull of the sharp stuff)
Offroad tire - All terrian or Mud terrain tire
Offroad tire / on road - 16-18psi
Offroad tire / off road - 10-12psi depending on wheel/tire width ratio
extreme tire - Something built for extreme wheeling like a Swamper SX or Swamper Bogger
extreme tire / on road - 12-16psi
extreme tire / off road - 4-6psi
When I ran 32/10.50 Swamper SX's on 8" rims I could air down to 4psi and run the trals all day without a problem.
Now I run 33/12.50 Swamper TLS Radials on 8" rims and I only go down to about 8psi on the same trails. (thinner sidewall)
*note* if you run beadlocks you can go even lower on the trail and not worry about losing a bead. But most beadlocks are also illegal to use on public roadways in most states...