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Tire Pressure question

14K views 21 replies 17 participants last post by  TJeepman 
#1 ·
Just put a set of Firestone Destination A/T's on my TJ. They are 33 x12.50x 15s. The primary useage is on the street or being towed behind our RV. When traveling we use the jeep on forest roads and are planning a trip to eastern Utah later this month. Anyone have suggestions for tire pressure for street use.
I realize that I need to air-down for sand, rough trails etc but the guys that installed the tires really had no suggestions and put 35psi in them. From watching the tires they seem to be slightly lifted on the edges but that may be my imagination.
 
#2 ·
I'd personally run that size tire at 28-30# pressure for street driving. I am running 33x12.5 Cepek Crushers on my 03 rubicon. Offroad in the woods maybe 16# or so, I think once I went as low as 12# but the Crushers are so good on grabbing I'll stay with 16#. Running 28# on the street the tires are all worn about the same amount-no feathering or cupping.
 
#3 ·
35 psi was crazy-high, they probably just read what was on the sidewall which is never the correct air pressure to use.

For a 33x12.50x15 holding up a vehicle with the weight of a Wrangler, 26 around town and 28 on the highway when loaded is appropriate. :)
 
#15 ·
Yeah..... what would the MANUFACTURER know about the correct amount of air to put in THEIR product??????

Seriously.... unless you are willing to put up a few MILLION dollars for liability coverage.... you should probably quit telling people to violate the manufacturers' recommendations....
 
#8 ·
I run what the vehicle calls for on all my vehicles. Only time I change the pressure is to go down the track or to go offroading. PSI recommendations are per vehicle weight on that particular axle. I know there are over thinkers on here that are thinking, well there are more square inches in a larger tire, so you don't have to run the same pressure to hold the same weight. Maybe so, but the recommended pressure works fine for me. Best way to make sure you are running the correct pressure is to check your footprint.
 
#14 ·
Read the side of your tire see what the max weight at max psi. I run 36" swamper iroks there rated 2860lbs @ 30psi. My front end weighs 1700ish divide that in half you get 850lbs each front needs to support. So I run mine on the high side at 20psi street, 4psi off road.
 
#18 ·
I should have added this additional information from Discount Tires to my above #16 post just above the information I posted on the air pressure...

"Maximum Load Limit - "635 kg [1400 lbs] (for example)"
This indicates the tire's maximum load-carrying capabilities when the tire is inflated to its maximum inflation pressure, as indicated on the sidewall. Max load is based on standards set by the National Highway Traffic Safety Administration (NHTSA)"

That was just above the information I quoted above already and provides further clarification.
 
#22 ·
Some people run the "max" cold pressure stated on the sidewall of the tire. The tire is designed to take that pressure and more (considering the tire will heat up with driving) without failing. These tend to be people trying to get better fuel mileage and are prepared to put up with the rougher ride.
From Recommended tire pressure - Transport Canada
The maximum tire pressure marked on the tire sidewall refers to the pressure required to carry the maximum load of the tire, and is generally not the same as the manufacturer's recommended tire pressure for your vehicle. To find the recommended tire pressure for your vehicle's tires, refer to the information label, which is usually located on the edge of the driver's door, or the door post or other conspicuous location.
Overinflation versus underinflation from Tire Tech Information - Air Pressure - Correct, Underinflated and Overinflated
Disadvantages of Underinflation

An underinflated tire can't maintain its shape and becomes flatter than intended while in contact with the road. If a vehicle’s tires are underinflated by only 6 psi it could lead to tire failure. Additionally, the tire’s tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tire to deflect (bend) more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. You would experience a significant loss of steering precision and cornering stability. While 6 psi doesn’t seem excessively low, remember, it usually represents about 20% of the tire’s recommended pressure.

Disadvantages of Overinflation

An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road. Higher inflated tires cannot isolate road irregularities well, causing them to ride harsher. However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures. The pressure must be checked with a quality air gauge as the inflation pressure cannot be accurately estimated through visual inspection.
I find that tires, inflated to the psi stated on the door placard, tend to wear more on the edges and less in the center. After all, that pressure on the door placard is a compromise between "wear", "handling" and "comfort". Inflating by 4psi more will likely result in more even wear. If the center starts showing more wear, cut back the psi by a couple. Keep monitoring via tread depth gage, at every tire rotation, and adjust psi accordingly.
Very Important: Tire pressures should be checked/adjusted when tires are cold per what the Owner Manual for the vehicle says. A tire at 33 psi cold, could show 34 psi when the ambient temperature warms up by 10F. From: http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=73
The rule of thumb is for every 10° Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower).
 
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