Tire PSI?

I have a 1999 Cherokee with 31x1050R15 BFG MT's. I primarily drive on the street. When trying to find the best PSI to set them at for the street, all I can find is a 50psi mark. Is that right, or is it too high?

Look on the door at the sticker that has the recomended tire psi on it, and run that. There also is the methiod of putting a line acrost the tire with some chalk then adjusting the air pressure so that the line wears off evenly.

got to a hardware store and get a big fat yellow lumber crayon, mark a fat wide stripe across the tread of 1 of the front and 1 of the rear tires
start w/ psi at 35 or so then drive around the block a few times,
observe the wearing of the crayon mark if the center is worn most psi is to high, lessen the psi in 2 or 5 psi incrementss until the wear is equal,

works w/ any size and flavor of tire and sets the tire for optimal wear performance..

w/ 33x10.5 on 7"rims i ran 25 in front and 22 in back

w/ 35s on 8s i'm runnin about the same
both BFG muds

I have Good Year MTR 31"X10.5"X15" and I run 30 PSI all the way around. My tires have about 58,000Mi and still in pretty good shape

The 50 psi on the tire sidewall is the maximum allowable pressure for that tire, to carry the maximum load it is rated to carry. That's much to high for normal driving.

On my MJ with 31x10.50s I ran 30 psi and they seemed to wear okay. 28 might have been better. I would not go below 28 if you do a lot of highway driving because of excessive sidewall flex. That's what did in the Firestones on Explorers. Firestone recommended a minimum pressure of 28 or 30m but Ford told people to run less so they'd get a softer ride. IIRC all the incidents involving blowouts of Firestones on the Explorers were on the highway after the tires had gone far enough to heat up.

The chalk on the tread method gives you the optimum pressure to keep the tread uniformly in contact with the pavement, but with tires significantly larger than stock that pressure may be too low to be run safely at high speeds.

Tire size above the OEM size is a bit tricky regarding pressure. I am running 30X9.5-15 up from 225X70-15. With the smaller tires, 28 PSI were perfect on city streets and 32 for long highway drives with good handling all around. However, the bigger tires rode a lot firmer on 28 PSI and yet are a bit squigly in the corner (I think it is due to tire side wall flex). I simply run them at 32 PSI both front and back all the time. A firmer ride? Yes, but the peace of mind in those long fast corners are worth it. I would say start at about 28 PSI and experiment up in 2 PSI incriments until you are comportable.

I just bought brand new 30x950 BFG mud terrains to replace my brand new 235/75 BFG all terrains, sold those to a freind, go figure. I have been told that the higher pressure you run at the better gas milage and tire life you get. I run 45 psi all the way around, Is that to much?

If the pressure is too high (above where the chalk says you should be), then you start to get uneven tire wear biased towards the center of the tread, because the edges aren't contacting the road that hard or at all. 45 is probably too much.

Do the chalk thing, thats the best way to find out. I have 31x10.50x15 BFG a/t ko's on my 99 and I run 32 lbs all the way around and have good wear all the way accross.

Pogiboy said:

I just bought brand new 30x950 BFG mud terrains to replace my brand new 235/75 BFG all terrains, sold those to a freind, go figure. I have been told that the higher pressure you run at the better gas milage and tire life you get. I run 45 psi all the way around, Is that to much?

Click to expand...

Yes ... MUCH too high.

You'll get better gas mileage that way, because you are not riding on anywhere near the entire tread. But you'll wear out the tires much faster, for the same reason. You're riding on a narrow strip of the center of the tread. Try the chalk line across the tread experiment described above and you'll see that the chalk disappears from the center long before it disappears from the edges of the tread.

With 30x9.50s you should be at 28 to 30 psi.

When I was on 225 75 R15's I ran them at 33psi all around and got nice even wear. Now that I'm on 245 75 R16s, I've been running them at about 28psi. From looking at them they seem to be wearing well, but I haven't had time to chalk them yet. I did get them used, and I've already worn off the high spots from the previous pressure that the guy had used, about 35psi. Now they're looking more even and run quieter (overinflated MT/R's only ride on the center 2 lugs...)