For Heavy Duty Four Wheeling
The pressures in this chart are suitable for most heavy duty four wheeling: rock crawling, mud, sand, some snow, etc., at speeds under 20mph. It should be noted that these pressures are suggested as a starting point only and, because of the many other variables involved like rim width, tire construction and weight distribution, one should experiment to find his/her ideal pressure.

Since 16.5" rims have no safety bead to keep the tire seated at the low pressures required for heavy duty four wheeling, they are not included in this chart and are not recommended. They are however suitable for light duty four wheeling at pressures ordinarily used on pavement.

General Rules
1) Increase pressure approximately 3psi for each 10mph over 20mph, until normal highway pressures are reached. For example: A 3000 lb. vehicle with 31x10.50R15 tires traveling at 40mph should have about 15psi in the tires (9 + 3 + 3 = 15).

2) Decrease pressure by approximately ½ for extremely soft snow. The exception to this is a 33x9.50R15 tire, which usually benefits from an increase in pressure.

3) Decrease pressure approximately 1psi for every two inches tire diameter is increased beyond this chart, assuming a corresponding increase in width of the tire.

Warning !!!
Use extreme caution when driving with partially deflated tires (any pressure other than the manufacturer's recommended pressure) whether on or off pavement. If you are traveling too fast while turning with partially deflated tires you may pull a tire's seal off of its wheel. You will instantly lose all the air in that tire, which may result in your vehicle rolling over or going out of control, causing death, bodily injury and/or property damage. The dangers of traveling with partially deflated tires are NOT LIMITED TO THIS EXAMPLE!

Deflation Tire Chart