PSI for BFG ATs

I've looked all over the tire and havent found a PSI marking. Id imagine 30-35, but I want to be sure. They are 31X10.5 R15

Its there. I run 40 on the road. The range on my 30's are 30-60psi

I think that the max is 40psi, but i run them at 25rear/28front on the street. The sidewalls are stiff, so the lower pressures make up for it. 30/33 for the highway.

tire pressure will vary from rig to rig because no two will weigh the same.

-Scott

rule of thumb for tire pressure-

find out what the front and back of your rig weighs, as it sits with you in it and so on

Look on side of tire for the max rating @ PSI <for the BFG AT I think it's 2295@50 PSI>

divide max weight by pressure it is rated at, sum is then used to divide into 1/2 the weight determined earlier <since each end of the truck has two tires holding it up>

2295/50=45.9 lbs supported per pound of air in the tire <I realize this is not strictly linear>

1500 pounds <approximate weight of the front of my Blazer, for example> divided by 45.9= 32.6 psi. Let's call it 33.

I usually run about 35psi all around on the Blazer since the weight front to back is pretty much the same by the time I am done loading it down. Working this formula lets you rest pretty assured you're not getting an undue amount of tread heating due to too low a pressure.

The best way is that you need to have a 4 psi increase from cold. If you have less then the starting pressure was to high not allowing enough sidewall flex. More and you started to low.
More info on my site about it.
http://go.jeep-xj.info

The pressure that you should run on any tires, as long as you areen't exceeding the maximum on the sidewall, is what is printed in the owner's manual, or the glove box, or whereever the manufacturer put it. With 31's, unless you have added lots of weight to the stock vehicle, you can probably go lower than the manufacturers recomendation. If you like a rough, stiff ride than fill them to the max, you'll get better gas mileage. I've run 22 front, 24-26 rear on my 30 x 9.50 BFG AT's for close to five years, and they are barely half worn.
YMMV.

Fred

Fred said:

The pressure that you should run on any tires, as long as you areen't exceeding the maximum on the sidewall, is what is printed in the owner's manual, or the glove box, or whereever the manufacturer put it. With 31's, unless you have added lots of weight to the stock vehicle, you can probably go lower than the manufacturers recomendation. If you like a rough, stiff ride than fill them to the max, you'll get better gas mileage. I've run 22 front, 24-26 rear on my 30 x 9.50 BFG AT's for close to five years, and they are barely half worn.
YMMV.

Fred

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Not true. The load is carried by the volume of air in the tyre. There is an optimum volume for each given load and conditions regardless of the tyre size. Remember I am talking about volume not the pressure. If you change you tyre size you have to change your pressure to keep the same volume. A large tyre will carry the same volume at a lower pressure due to the larger area. Conversely, the smaller the tyre the more you have to force the same volume in, so the pressure goes up.

Gojeep said:

Not true. The load is carried by the volume of air in the tyre. There is an optimum volume for each given load and conditions regardless of the tyre size. Remember I am talking about volume not the pressure. If you change you tyre size you have to change your pressure to keep the same volume. A large tyre will carry the same volume at a lower pressure due to the larger area. Conversely, the smaller the tyre the more you have to force the same volume in, so the pressure goes up.

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Sorry, Mate, but your "not true" is not true. By self-definition, a larger tire has a larger volume. The concept you are trying to explain is valid, but your explanation doesn't hold up. It's pressure that carries the load, not volume. What you are implying is that with a larger tire you can squeeze more air at atmospheric pressure into the tire volume at an equal or lower pressure than with a smaller tire. That equates to pressure, not volume, being the governing factor.

The owner's manual for my '88 lists multiple tire sizes. They recommend 33 psi for 195/75R15s, and 30 psi for 205/75s, 215/75s, and 225/75s. (These are listed as full load pressures.) In short, the smaller tire requires the highest pressure.

Once you go above the sizes listed in the owner's manual, the best way to find the pressure that will optimize tread life is the chalk line. Draw a straight line across the tread with a piece of white or yellow chalk. Then drive down a straight length of road (or, ideally, empty parking lot). A quarter to half a mile should do it. Stop and look at the line. If it's worn evenly, the tire pressure is correct. If the line wears off in the middle before the shoulders, the pressure is too high. If it wears off the shoulders before the middle, the pressure is too low.

Because larger tires need less prsssure to carry the same weight (whether you arrive at that through your reasoning or Burntkat's), it is extremely unlikely that the optimum pressure for a 30 or a 31 on an XJ would ever be 30 psi or greater. Rim width plays into the equaltion, because narrow rims tend to pull the shoulders up, requiring a lower pressure for even tread contact. The optimum pressure may turn out to be 28, 26, or even lower. And will be lower still for larger tires.

Then you have to decide if you want to run that low a pressure to optimize tread contact, or avoid heat build-up by running at 28 to 30. Remember, Firestone's argument with Ford was that Firestone told Ford to run the Exploder tires at 28 psi, and Ford told customers to run them softer to get a better ride. Heat build-up from prolonged high speed driving was what killed most of those tires.

There is certainly no need to run a tire much larger than stock at a higher pressure than stock.

Man, there are some big words floating around over tire pressure. The guy asked a simple question of what tire pressure to run in his 31 Bfg's


-Red

Redcbr007 said:

Man, there are some big words floating around over tire pressure. The guy asked a simple question of what tire pressure to run in his 31 Bfg's


-Red

Click to expand...

28 psi

Happy now? :laugh3:

Redcbr007 said:

Man, there are some big words floating around over tire pressure. The guy asked a simple question of what tire pressure to run in his 31 Bfg's

Click to expand...

Are you implying everyone is full of air? (hot ). Do the chalk thing: fill up tires and draw a thick line across, drive around the block and check which part of the line got worn out. Then adjust pressure as needed till you get an even wear on the line.

Thanks all for your responce. I will try out the chalk deal and see how that goes.

Kejtar said:

Are you implying everyone is full of air? (hot ). Do the chalk thing: fill up tires and draw a thick line across, drive around the block and check which part of the line got worn out. Then adjust pressure as needed till you get an even wear on the line.

Click to expand...

The problem with the chalk method is that it does not at any time take into account the pressure change that occurs during driving where the sidewall flexes. This will alter the pressure quite a lot depending on the roads being driven and starting pressure etc. You can get you chalk line perfect and then go a drive and then do it again quickly and it will then be wearing it out in the centre due to the rise in pressure from the tyre heating up.
The 4 psi rule is not something that I came up with at all but from a leading tyre technician that writes for a national 4wd magazine. I came across this gem after suffering 7 punctures in one 7,000 km trip through the Outback due to running the wrong pressures. I have only suffered two in the last 8 years since where it was staked and not from stone punctures. I have been getting even wear ever since and 70,000 kms from them which I didn't get when using the chalk method.

Eagle I still have problem understanding the difference. If I need say 100 litres of air to support a corner of the car, it would mean in a tyre of 50 litres it would be at 2 psi. If the tyre was only 25 litres it would take 4 psi to get that volume of air into the tyre. It still stands that the larger the tyre the lower the pressure which you also support. Can you help me see what is wrong with this explaination?

I'm no expert on anything at all ... but I do have an opinion. In my mind, I don't think you can do anything other than the chalk method to find the "correct" tire pressure ... there is just too many variables involved ... especially with a modified vehicle. After finding the best wear pressure, one then has to factor in if it's a safe pressure for the type of driving you are doing ... and whether or not you just plain like the ride.

Personally. I like a softer ride on a daily basis because the roads around here are kinda rough and my suspension is a little stiff. I'll forego a little bit of optimal wear so I'm comfortable driving. I'll suspect it would not be a safe pressure for high traffic, high speed driving. When I went to Paragon and drove the interstate ... I aired the pressure up to reduce heat build up and noise ... and get better gas mileage. I'm sure the "wear pattern" was blown out of the water, but it suited my current "need" and was within the manufacturers safe operating range.

I wouldn't get too hung up on tire pressure. Find where you get optimal wear and adjust to suit your needs. The most important thing to do to get good wear is to keep your alignment correct and run good shocks.

JMHO ... Les

I drive in LA area about 120 miles a day and my tires always wear nice and even and I used the chalk method to determine my pressure. And I don't really suffer any puncture issues. Also I have gotten over 55K miles on my second to last set of tires and I still had a fair amount of tread left when I replaced them (this previous set my speedo was off too often so that I don't know how many miles I have on them, but I'd guess ~30K and I got about 1/2+ tread left).
You do have a point about the rising pressure though, but I took care of that by doing the chalk thing after driving for 15 minutes or so.

Gojeep said:

The problem with the chalk method is that it does not at any time take into account the pressure change that occurs during driving where the sidewall flexes. This will alter the pressure quite a lot depending on the roads being driven and starting pressure etc. You can get you chalk line perfect and then go a drive and then do it again quickly and it will then be wearing it out in the centre due to the rise in pressure from the tyre heating up.
The 4 psi rule is not something that I came up with at all but from a leading tyre technician that writes for a national 4wd magazine. I came across this gem after suffering 7 punctures in one 7,000 km trip through the Outback due to running the wrong pressures. I have only suffered two in the last 8 years since where it was staked and not from stone punctures. I have been getting even wear ever since and 70,000 kms from them which I didn't get when using the chalk method.

Eagle I still have problem understanding the difference. If I need say 100 litres of air to support a corner of the car, it would mean in a tyre of 50 litres it would be at 2 psi. If the tyre was only 25 litres it would take 4 psi to get that volume of air into the tyre. It still stands that the larger the tyre the lower the pressure which you also support. Can you help me see what is wrong with this explaination?

Click to expand...

ok...ok..i'm full of hot air...i have 31/12.50X15's on my rig on 10inch wide rims....i run 22psi in the front and 20psi in the rear...how i do my setups is wet a spot on the paved or concrete road..drive thru it slowly onto the dry...look at how much patch is touching the ground...lower it down till you get a full patch!!...i've done that for my last 4-5 sets and get amazing mileage outta my tires...Len

interesting....

i just look at the side of the tire and adjust for a slight bulge. been running bfgs on our cars for my entire life. I think that getting into the theory behind pressure adjustments is a little too much. i do agree with crash tho. :wierd:

sidewall stiffness is another factor too.

use chalk, water, or both. have fun, be safe.

BTW, those guys at discount tire co. always set them to 40psi when i get new meats. rides like crap.

g'luck

25/28 gets my vote