Tire Pressure

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Bought by His blood, kept by His power
Oct 12, 2003
Middleburg, Florida
What is the normal street tire pressure for a BF Goodrich TA/KO 285, 75R 16? I checked them after they were installed and the pressure was 48 psi........ is that high?

Wayne S
It sounds to me as though the tire guy seated the beads and forgot to let some air out :whoops: . Stock cold tire pressure is 32 psi. I run 32 psi in my 305/70/16 GY's. It seems to be fine for them. Some members run higher pressure.
I read some interesting tire pressure theory a while back, it may have been on 80's cool, I can't remember for sure. It goes like this: Ideal tire pressure should give you a 4 psi spread between cold and hot. In other words 32 psi cold with a stock tire should read 36 psi when the tire is hot. I checked that out on mine one time. I set the stock tires to 32 psi cold in the morning and went for a prolonged high speed run(50 miles at 75mph +/-). At the other end the tires had 36 psi. So it made sence to me.
Supposedly if the spread is less than 4 psi the the tire is over inflated and if it is greater than 4 psi the tire is under inflated.

Tire experts? anyone?

I usually run 35 on the street.

As for methods, Another method is to chalk the tires. Make a line all the way across the tire then drive down the driveway. If all of the chalk is removed evenly it is correct. If the center wears it is overinflated, if the centers stays but the side wear it is under inflated. But if you have too fat of rims or too skinny of rims for your tires, you can't tell anything about pressure this way.

On sports cars you tag the sidewalls with chalk and drive the corners hard. If you tire is rolling over onto the sidewall you need more air. You want to run the minimum psi that prevents rollover.
One other thing, if running C02 don't put too much air in as tire pressure increases significantly with C02 vs. normal air. 25psi CO2 cold netted me 38 psi after driving 100 miles at 80mph.
45 is too high for any tire. The inflation pressure on the side of the tire is max inflation on any vehicle. The manufacturers recommended inflation is on the drivers door jam sticker. This should be adjusted for radically different tires than those recommended by the manufacturer and for vehicles equipped very differently, but that should be the starting point, not max inflation.
I want to say that the stock tire pressure is either 32 or 35, but I can't remember off the top of my head.
27Oct2003 (UTC -7)

I run 35 psi (cold) on my UZJ-100's BFG A/T, and also on the HDJ-80's GoodYear MTR. For me, it's the right pressure for the 'Cruiser weight, steering and handling and on-road traction.
I make my tire pressure decisions based on the load and/or vehicle weight based on a life with heavy construction vehicles. There is no one inflation fits all unless all the vehicles are the same weight and tires. An underflated tire gets hot. Why? There is not enough psi to support the weight which results in a lot of tire flexing and heat. Can anyone say Ford-Goodyear. I run a system in my tires called Smartire which wirelessly monitors tire pressure and temps. It sets off an audible and visual alarm in a tire is going down etc. This also allows for monitoring an individual tire to see if one is hotter than the others and could use a little more air. (like Eric's procedure)
I generally run 55 to 60 psi in 285's in a heavy vehicle, with no wear or handling issues. My range is outside of the 4 psi suggested by CDan. It runs from 52 psi in the mornings to around 62 psi after things have warmed up.

Based on your narative, it appears that the theory I read about does not hold water. At least not for heavily loaded vehicles. I wish I could remember the source.

[quote author=ginericfj80 link=board=2;threadid=6811;start=msg55903#msg55903 date=1067306399]
One other thing, if running C02 don't put too much air in as tire pressure increases significantly with C02 vs. normal air. 25psi CO2 cold netted me 38 psi after driving 100 miles at 80mph.


I don't agree with your statement above regarding running CO[sub]2[/sub] in your tires. CO[sub]2[/sub] has the same expansive properties as "normal air". You would NOT see an appreciable difference in running "normal air" vs. CO[sub]2[/sub]. Infact, when using CO[sub]2[/sub] regularly, you DO NOT put moisture in your tires like most service station air systems that usually don't have driers in the lines. So you won't be introducing another variable, the moisture in "normal air".

My tires have max of 65 for one and 50 for the other. Some tires have 85 for max.
Running 32 on a tire of 65 would be running too low wouldn't it???
I thought that the reading posted on the tire was the recommended tire pressure, not max. My tires said "50 psi cold" so i thought they required 50 psi. Well the cruiser rode like a horse drawn wagon. I thought if i let the air down to 35-37 they would wear quickly. Did the chalk test and found out that they were wearing just fine. So now they are at 35 psi and the ride is mucho better! :)

The answer to that question is " it depends ". The rated max pressure is simply the rated pressure at which the tire can be loaded to its max sidewall load rating. This speaks to Photo's comment in which he points out the relationship between inflation and heat. The higher a tire is inflated, the less flex and therefore the less heat it will generate.

Getting to your question, the factory inflation pressure is a good starting point for a tire that's the same size and type. Go to larger tires, and theoretically you'd lower the pressure so the footprint of the tire remains approximately constant for optimal handling. Another theory I like best is to adjust the pressures based upon use/load. For instance, I run 35 psi for ordinary driving. While towing and/or laden with gear and people I run 45-50 (my tire max is 55, I believe).

On a side note, if you have two different types of tire on your 80 (65 max on some, 50 for other), you're putting needless wear on the VC and/or center diff as the rolling circumferences are likely different. Just last week, I suspected this to be the case on my Mother's Sube, which is across the country from me. I recommended new tires and the binding/noise indeed went away. Full time systems can be finnicky about that.

Finally, I've tried the chalk line method and found it inadequate. Through quite a range of pressures, the chalk line wore the same, plus some tread designs (muddies for example) have a lot of "tread squirm" on the center blocks that make the chalk line appear to wear quickly vs the more stable shoulder blocks. Tough call, but the best is to pay attention to pressures and never underinflate for road speeds.

For a given psi and weight vehicle, you'll get a certain footprint area. Pounds weight divided by Pounds Per Square Inch = Square Inches of tire on the road. If you add weight to your vehicle, add pressure to the tires to keep an even, symmetric footprint. A wider tire, properly inflated, should give a larger footprint, so less psi. My 265s have the same diameter and tread width as my wife's 270s, so same pressure.

I do a lot of highway driving, and I like to run a little extra pressure. I typically wear out the centers of my tires early because of that. I suspect if I did more around town driving I'd wear the edges more and it'd even out. I suspect that's what's happening for the guys who overinflate but get even wear. These trucks tend to wear out the edges of tires quickly with even mildly aggressive driving.
One thing that everyone seems to have missed is the tire's makeup! How many belts, what type of belts, the load range, the sidewall strength and rubber compound, these will all play into how pressure is displaced accross the contact patch. Then factor in lift, accessories, cargo and passengers and we're all going to be different in what the ideal pressure would be. And this doesn't even take into account personal preferences.

When I first mounted my 315's I started at 35psi. It drove terrible so I upped it to 40 and then to 45. As a side note I was having an intermitent vibration from the rear right tire. Myself and a coworked visually check the tire several times and gave it the kick test ::). It wasn't until I checked the pressure that I realised I had grabbed a nail. It had only 8psi in it and I had driven it that way almost 2000 miles. And it looked fine to me.
Ooh, don't get me started on tire/road interfaces.

It's true that a larger tire with the same PSI will have the same square inches of contact patch. What begins to change is the shape of the contact patch itself. It moves from an oval elongated in the direction of travel (highly stable directionally, max for/aft traction) to either round, or in the extreme elongated laterally (highly unstable directionally, minimum for/aft traction) as larger tires are fitted. So, I'm advocating a drop in PSI for larger tires to maintain a footprint that provides proper handling.

It's quite easy to put larger tires on and REDUCE your available traction for fore/aft traction (braking and accelerating). Most of this traction comes from the length of the center 50% of the tire contact patch. Shorten this area, and you don't make up for the loss of traction with a wider tire (because a the same pressure there's no more tire in contact with the road, but the shape HAS changed). Let air out and you'll elongate the contact patch a bit to make up for it.

Well since nobody is mentioning the reason for different maximum pressure ratings, I will. The reason that some tires (truck in this case) have higher pressures have to do with their max load rating. An E rated tire will have a higher max pressure than a C rated tire. If you look at the sidewall of the tire you will see that they will say somthing like "max load 2822lbs @ 65psi" This means that the tire can take a static loaded weight of 2822 at max pressure. You can run lower pressures with a lower static load.

There is no need to run 50-65 psi in the LC. All you are doing is increasing tire wear and giving a harsher ride. As Doug said, as you go to a larger tire diameter, you actually would normally lower the tire pressure. My suggestion would be the following:

Start with the stock pressure:

For long trips, increase by 2-4 psi,
For carrying a load, increase by 4-8 psi
If you have winch, sliders, etc, increase by 4 psi over stock.

So long trip, fully loaded, sliders, winch, bumpers, etc, I would go 10-12 psi over stock.

In no case would I go below stock pressure for road use. My concern is that given the high sidewalls, you could potentially roll the tire on the sidewall when cornering with low pressures.

please forgive the ignorance, but what's all this about a stock pressure spec (as found on the sticker)? Would that not be only relevant for the OEM tires? Wouldn't a different tire size or design make this number irrelevant?

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