Tire Pressure

[BusaDave9]

A question that often comes up is "How much air pressure should I put into my tires?"

This really depends on too many factors to give a specific pressure.
But here is how you can find the correct pressure for your jeep: Take some chalk and draw a line across the tread, from the inside of your tire to the outside. Drive a few blocks. If you wear off the chalk evenly then the tires are at the correct pressure. If you wear the outer edges first then you have too little pressure. If you wear the inner part of your tread off first then you have much pressure.
You could do the same test with masking tape but you would have to drive further to wear it off.

[Old Dog]

Great advice Dave.

But I think There should be a cut off somewhere, even if you don't have full tread contect yet.
Maybe 20 lbs for on road use so the sidewalls don't get to flexy in hard turns.

[BajaEdition]

thanks for the post, I use the method and found 28 pounds is a good figure for a TJ, and Cjs depend on the type and year.

[Thysis]

Never would have thought of that. Thank you!

[crust]

I have used that method before, but I feel you only want to go straight. Turning will cause the tread to roll and give untrue ware on the outer edges of the contact patch. You can also start at the max rated pressure for your tire and reduce presure in small increments until the shoulder (where it turns from tread to side-wall) of the tread contacts the ground. remember this is just a starting point. I usually go use a parking lot that is pretty level at a large store and just drive back and forth until I get the right pattern. On some vehicles it is possible to have a higher pressure in the front than the rear tires when unloaded.

I have spent too much time messing with this since when ever I get a new ride the PO tends to put LT tires on a small truck or jeep and as directed they fill the tire to max rated pressure.

[MN CJ7]

I have read the Large Chalk Line on the tires and see how the chalk line wears as you short drive. Past readings show this to be a good idea and it works. We also need to consider that tires are PSI, pounds per sq inch. So the larger the tire and the larger the foot print on gound, the lower the tire psi can be and still support the vehicle weight. We also need to consider the CJ is one of the lightest vehicles and is 2200# or so.

I was running 32 to 33 psi like I do in my passenger tires. I have after market springs and it rode like very stiff TRUCK. I lowered it at 1 psi all the way to 22 psi. The lower 20's give a softer ride but the steering is more mushy. I did not ck the hwy mileage with lower pressure.

I use 28 psi in front and 27 psi in the back and seems to work well on 33x`12.5" radials. HWY miles I am always in the 21 to 24 mpg range. The softer tires yields a much smoother ride. Still a truck ride but tollerable.

The JEEP 1979 Owners Manual, pg 23
The 1979 CJ with L-78-15 tires would use 20 psi with general use and hwy use. If long sustained hwy miles JEEP suggested 24 psi. I was a bit taken also and it appreared to be a bit too soft for me. BUT is should make you feel a bit safer if you would like to drop your pressure from 33/35 down to the mid/upper twenties.

9-15 Tire Size has 30 psi for sustained HWY travel and 20 psi for general use and hwy travel too.

[BusaDave9]

Originally Posted by MN CJ7 tires are PSI, pounds per sq inch. So the larger the tire and the larger the foot print on gound, the lower the tire psi can be and still support the vehicle weight.

True, Because tire sizes vary, this is the main reason for this thread. If everyone used stock tires they could use the manual for air pressure.
Trucks use large tires to distribute the weight better. If a jeep is fitted with oversized tires those tires are made to hold up much larger trucks than a CJ. You don't want to fill up over-sized tires with the air pressure recommended for a big truck.

[jeepy]

I like the ideas I am hearing. I know what it needs to be off-road (5psi), but on road, I tend to set it at different pressures.

Keep in mind I run 35x13.5 bias ply tires.

If I want a soft ride and I just don't care about anything else. I set it to the low 20's.
But If I know I am going to be doing a lot of driving on pavement, I crank it pretty high up to about 33-34. The reason I do that is to try and protect the outside edges of the big lugs. They seem to lose their edge the fastest so I try and protect them by making the tire ride more on the center. Of course if you go too extreme you wear out the middle.

I will play around with the chalk idea and see what kind of results I get. Maybe for a longer test, I could spray paint a section of the tire. Wouldn't that look great going down the road!

This test just may have to wait for warmer weather though. No top.

[Jason S]

1) I'm fairly certain that in 1979, national highway speed limit was 55 mph max. Tire manufacture and technology has also improved quite a bit since 1979. Something to keep in mind when following suggestions from the owner's manual in regards to handling.

2) This method WILL NOT work if your tires are already unevenly worn from inproper tire pressure. Never exceed the max cold pressure, which is usually 50psi LT tires. On average 30-35psi is a good target, most tire shops will air a tire to 35psi unless requested otherwise by the owner.

Never make final tire pressure adjustments after you've been driving around for a while. Always make adjustments when the tire is 'cold'.

Your tire's pressure is never constant. As you drive around your tires warms up. When it does, the pressure increases. A tire naturally loses air (pressure) at a rate of approx.1psi per month. A tire will also gain/lose about 1psi for each 10 degree change in ambient air temperature.

On a racetrack (of any variety), tire pressure is a science, 0.1psi is important. On the street, it's just a ballpark. Keep all four tires at about the same pressure give or take 1 or 2psi, and you will be fine.

Now, if we must discuss tire pressure may I recommend that we discuss how far to air tires down when wheeling.

[BusaDave9]

Originally Posted by Jason S Now, if we must discuss tire pressure may I recommend that we discuss how far to air tires down when wheeling.

Airing down for off-roading? That's an interesting topic and there is no one definitive answer. Some people go down to single digits.
That sounds like a separate thread. Start one up Jason, otherwise I might in a couple of days.
That brings up issues like: do you have bead-locks? do you have an on board air compressor? What kind of terrain: rocks or mud or dirt trails?