How many PSI new 30x9.5x15 tires XJ?

I just got some new Futura Dakotas 30x9.5x15 at Pep Boys(no lift, seem to fit fine)tires, I noticed they put 45 lbs in tires, they max at 55 lbs. I questioned this as I used to put approx 32 lbs in old tires(215's). I thought you were supposed to heed vehicle mfgrs. instuctuions regarding tire pressure, who's right? It's a 92 XJ 4.0 A/T, Thanks, Heinz
P.S. Should I do the old chalk test on tire tread to see wear?

Always go off the spec printed on the tyre sidewall, as that is where they are designed for general use. This does not account for "airing down" at the trailhead to gain floatation or for better "griptions," but for daily street use going with sidewall figure +0/-10% is a safe bet. Going too low on pressure will result in inadequate contact patch and accelerated sidewall wear, among other things. Remember that the sidewall may be an important structural element of the tyre, but it is not a treadwearing area, and will run thru in a hurry if it rides on the roadway at high speed...

5-90

You should follow manufacturer's instructions, unless the manufacturer is Ford and the vehicle is an Explorer. But -- Jeep never put 30x9.50s on an XJ, so there is not manufacturer's instruction to follow.

I run 30x9.50s on my '88 at 30 psi. I actually think that's a bit too high, but I only put them on when I head out to a trail ride so I don't pile on enough miles to see if they're wearing evenly -- and I bought them used, which makes it even harder to assess what's happening.

Try using 30 psi, and watch the tread wear. If they seem to be wearing more in the center than toward the shoulders, drop it to 28 psi. Don't go below 28 or you'll become an Explorer wannabe.

Heinz,

I went through this same question a couple years ago. I learned the pressure on the sidewall is the "maximum" pressure you should run the tire when it is "cold". If you have the max pressure in the tire then you can carry the full weight rating also on the sidewall.

If you have a vehicle which weight rating less per wheel then the car companies air down. For discussion sake lets say the tire can carry 2000 lbs but you vehicle only requires a tire to carry 1700 lbs. The car companies do air down but they also have to look at ride and load carrying ability.

When I purchased the BFG all terrains for my 88 XJ, I checked the BFG site and it said 40 psi for my XJ. If I check the owner's manual and glove box cover it says to run the tires at 30 psi.

On the stock tires they had a max sidewall pressure of 35 psi, the BFG has a max sidewall pressure of 50 psi. With more plys the tire can hold more pressure. I run my BFG ATs at 40 psi and with 25,000 on them no sign of unusual wear and they ride well.

So see if Pepboys has a tire selector and a recommended pressure for your XJ. It a function of load carrying capablity. If you run under inflated you will generate heat, reduce MPG, and reduce tire life.

Martin

In my experience, 30X950 BFG ATs will wear faster in the center when run at 32-35 on an XJ. There do exist load versus pressure charts for tires; however, I can't seem to find a recent one. I'm presently running at 28 psi.

Do the chalk line test. I run 28 p.s.i for 31x10.5 on my CJ7 and 30 p.s.i. for 30x9.5 on my XJ.

Floatation type tires (31x10.5x15 etc.) are intended for larger vehicles that carry more weight. My tire store put in the max p.s.i. on my GoodYear MT/R's and it road like it was on solid steel tires.

To do the chalk line test rub some chalk across the full width of your tire treads, front and rear. Drive 10-15 feet, then check the chalk line. If it is gone only in the middle of the tread air down a bit and repeat. If it's gone only at the edges air up some. Your goal is to have the line disappear evenly from the tires. Of course, add air for towing and heavy loads. Underinflation causes sidewall flex and overheating ( blowouts! ). Overinflation causes rapid tread wear in the middle of the tires and reduced street traction.

Tim

Here's an article primarily about sand pressure; however, he does talk some about street pressure in 30X9.50 tires. He says he runs 26 psi on his Exploder with BFG A/T tires.

http://www.4x4now.com/sfjun96.htm

5-90 said:

Always go off the spec printed on the tyre sidewall, as that is where they are designed for general use. This does not account for "airing down" at the trailhead to gain floatation or for better "griptions," but for daily street use going with sidewall figure +0/-10% is a safe bet. Going too low on pressure will result in inadequate contact patch and accelerated sidewall wear, among other things. Remember that the sidewall may be an important structural element of the tyre, but it is not a treadwearing area, and will run thru in a hurry if it rides on the roadway at high speed...

5-90

Click to expand...

Right but realize that the ratings are usually the MAX load...Unless you're rig is weighing in at the MAX load rating on the tire, you'll want less.

My XJ tips the scales a hair over 4k with me in it. I run 32 front and rear and have excellent wear, YMMV!

RJ

The sidewall is the MAX load. TOZOVR is right. My 31 BFGs say 55 max. What I did to get the ideal contact patch:
I found a really smooth surface (non-brushed concrete) and laid a piece of white paper down. I drove over it, and the dirt on the tires transfered to the paper. I noticed that the edges of the tread didn't contact, so I reduced pressure.

It's a combination of rim width and pressure. the skinnier your rim the less pressure is needed to make it a full contact patch. you generally want the highest pressure that gives you a full contact. Also higher pressure gives a bumpier ride, you might want to take that into account when deciding. I run mine at 35 in the front and 33 in the rear.