[cary]

This has come up a few times before, but there has never been a clear resolution to the question, what air pressure should I run in my tires? I have been looking at this and believe I have most of the answer, but lack a little bit of data. Let's see if we can flesh it out.

The issue: How do I figure out the air pressure I should run when changing tire sizes/pressure?

Answer (Sort of): You have to start with the baseline pressure, load rating, and tire type. In this case the 80 calls for 32psi with a P-metric (this is important) 275-70-16 tire. This size and type of tire has a load rating of 114. The important information is that the tire has a load rating of 114 and is p-metric.

Now you have to look at what tire you are going to be using. Most of us are going to be using LT (Light Truck) tires. This is where it starts to get a bit sticky. The load capacity of a tire is figured at is maximum inflation pressure. For most P-metric tires this max pressure is 35psi (there are XL rated P-metrics with higher pressure limits). For LT tires the max pressure is 50 psi (load range c), 65psi (load range D), and 80 psi (load range E).

Now it gets even a bit more sticky, the only thing that letter load range capacities are used for any more is the MAX inflation capacity. Other than that they are worthless. Any tires maximum load is based upon its numerical load rating (for us about 109-122). See the load table on this page:

http://securitydriver.com/aic/stories/article-116.html

Are you confused yet. It gets worse. For a given sized LT tire, it will be able to carry the same load at the same pressure, no matter what it's load rating. So a 109 load range tire can carry as much weight as a 122 tire when inflated to 30psi (random number). What the higher load ranged tires do is allow you run higher air pressures as they have stronger sidewalls to withstand higher pressures. So when you hit the limit of set load range tire (say a C) at 50 psi, by going to the next load range tire, you can carry more weight because you can set it at a higher pressure.

Now the real problem. The load capacity curves for a P-metric tire is completely different than it is for a LT tire. This means that a P-metric tire inflated to 30psi will have a completely different load capacity (higher) than a LT tire of the same size a the same pressure. Then to make it worse, when using a P-metric tire on a SUV you have to adjust it's load capacity by multiplying its rated load by 91% (this is a safety factor calculation). See these links discussing converting from P-metric to LT tires.

http://www.nittotire.com/assets/safe...oad%20range%22

http://www.yokohamatire.com/pdf/tsb-...oad%20range%22

http://www.tirerack.com/tires/tirete....jsp?techid=70

So how do we figure out how much pressure to run. Well we start with the 114 load capacity of the oem sized P-metric tire. On the chart we see this is 2640 pounds. We then take this 2640 and multiply it by .91 to get the load capacity since this is SUV. We come up with 2402 pounds. So the rated capacity of the OEM tire is 2402 pounds at 35psi. We do not want to replace the tire with one that has a lower capacity. In this case we do not want to go below a load range 111 tire if it is a LT, or 114 if it is a P-metric.

So here is where it gets problematic. In order to properly set the pressure of a tire, you need to know it's rated load carrying at different pressure. Because load capacity is based on max tire pressure, these charts are broken down into P-metric and LT charts, and then by tire size. There are charts out there that show what at tires load capacity is at different pressures. So there will be a chart that shows the capacity of the 275/70/16 P-metric tire at 35, 30, 25, and 20 psi. Because the factory pressure is 32psi we know the actual capacity of the tire will be slightly lower than the 2400 pounds at 35psi (2300 pounds or so).

What you then do is take that 2300 pounds, and go find your LT tire on the load capacity chart. You then want to find the inflation pressure that supports about 2300 pounds. This is your proper pressure. For a LT275/70/16 the pressure will be about 45-50 psi. As you put on taller tires the required pressure will go down, as taller tires support heavier loads at a given inflation. So a 285/75 may only need 40 psi, a 315/70 35 psi.

The problem is that I can not find the inflation charts to cross reference the pressure. Does anyone know how to get them?

An additional factor to take into account, if your truck has lots of armor and other stuff that makes it heavier, you will likely want to increase your pressure 5-10 psi over the base.

If someone has the precise numbers for the charts, it would be great so we could come up with exact numbers.

[-Spike-]

That all sounds good, what it fails to take into account is that the tires with higher load ranges are expected to be run at the higher pressures, and the carcass is probably designed to be run at those higher pressures- meaning the tread will conform to the road and the sidewall will flex as designed. Taking a tire designed to run at 100 psi with 10,000 pounds on it won't perform properly inflated to 15 psi with 1500 pounds of load. I know the ratings vs. psi aren't that linear, but you get the idea.

-Spike

[Doc]

Oh my GAWD!

Can't you just put in 10 lbs less than max and just drive?

[CJF]

For normal, day to day driving, would you ever not just want to make sure (via the chalk method, or whatever) that you're getting very even tire wear across the tread???

[-Spike-]

Originally Posted by CJF

For normal, day to day driving, would you ever not just want to make sure (via the chalk method, or whatever) that you're getting very even tire wear across the tread???

Exactly, it's not complicated. The tread should be contacting the road evenly. Chalk or paint works for starters, then monitor tread wear for long-term effects. Buy tires meant for use on the vehicle for the conditions the vehicle is likely to see. Off-road tires follow different guidelines because treadwear and highway manners are secondary to performance off pavement, and expecting a perfect ride and 50,000 miles from them is silly.

-Spike

[pulse98]

How about this tire contact patch pressure sensor?:
http://www.sensorprod.com/static/tire_tread.php

www.mcmaster.com has them for about $70 for a 11" x 12" sheet. One is part number 31705K631 . Search for that pn on their site and you'll see the description and picture.

You can cut them into several 1 inch or so width strips. This way, you can get several passes, one pass per different inflation pressure. I am guessing that you want the inflation pressure that will give you the most even pressure distribution.

[cary]

Originally Posted by CJF

For normal, day to day driving, would you ever not just want to make sure (via the chalk method, or whatever) that you're getting very even tire wear across the tread???

The problem is that the Chalk method only works on brand new tires. Almost immediately the tires will wear in so that an overinflated tire will sit flat on the road. The proper way to do it is drive and then use an infared temp gauge to determine what works for each case. Since we don't all have access to these, I would like to figure out a more precise way of establishing pressure.

[Nay]

I run the amount of pressure where the tires ride and handle the best. For trxus MT's, that is 30-31 PSI. Great onroad and handle the washboard perfectly. Go up to say 37 and the ride and handling go to crap, and washboard is undriveable.

There is no chart that is gonna help you here - it will depend on the tire.

Nay

[CJF]

Originally Posted by cary

The proper way to do it is drive and then use an infared temp gauge

Hadn't heard of that; that's cool!

Curtis

[cary]

The infared is a great way for more than just inflation. On a track car it not only lets you measure if inflation is right (you measure across the tire in three places, inside outside and middle), but if your camber is to little or to much.

[BrettinSanAntonio]

Hmmmm...OK. What do I run 315, BFG MT's at? Right now all four are at 38psi.

[DanKunz]

I am running my MTRs at 40 on the road. Ride nice.

[BrettinSanAntonio]

Originally Posted by DanKunz

I am running my MTRs at 40 on the road. Ride nice.

Sweet, thanks.

[Grench]

Originally Posted by DanKunz

I am running my MTRs at 40 on the road. Ride nice.

I'm running my 285/75R16 D rated MTRs at 65 all around when I'm in town. The ride is harsher, but it handles well. The ABS kicks in more often... but the mileage is a touch better. Its all a trade off.

I've been thinking of cutting back to 50-55 psi.

[-Spike-]

I suspect that higher pressure should equal better gas mileage. The trade off, if over-inflated, is poor tread wear equaling more money spent on tires.

-Spike

[Grench]

Originally Posted by -Spike-

I suspect that higher pressure should equal better gas mileage. The trade off, if over-inflated, is poor tread wear equaling more money spent on tires.

-Spike

I've been keeping an eye on it. 65psi on GY MTR load D tires is "maximum" pressure. They're wearing even side to side, tire to tire within 1/32" so far.

This is, IMHO, a good pressure for these tires for city/highway driving during summer months. Air down for winter and don't drive these on gravel for any distance at this pressure. Washboards will wipe you out as the tires have nearly no flex under full inflation.

[-Spike-]

Yes, I believe most tires are good to run at less than the maximum, for the best handling, comfort, and wear. Especially if they're way over rated for weight.

-Spike

[Nay]

Originally Posted by Grench

I've been keeping an eye on it. 65psi on GY MTR load D tires is "maximum" pressure. They're wearing even side to side, tire to tire within 1/32" so far.

This is, IMHO, a good pressure for these tires for city/highway driving during summer months. Air down for winter and don't drive these on gravel for any distance at this pressure. Washboards will wipe you out as the tires have nearly no flex under full inflation.

65 PSI

I have found that BFG's need more air pressure to handle well...usually 35 and above, but then they ride harsh over every little bump, but the MTR's? You must float all over the road at that PSI. Do they even touch the ground ?

When you go to a bigger tire with heavy sidewalls that gives you the option to run a reasonable PSI without sacrificing handling. My trxus are load range E and they ride better around town at 31 PSI than the 285 BFG AT's they replaced without handling degradation. They just soak up the big and small stuff so much better. I can't even imagine 65 PSI, but what works for you works for you .

Nay

[Photoman]

Don't know if this will help or not, but here is a product that I used called Smartire. The sensors mounted to the wheel and sent information to a display I had on the console. It gave tire pressure and temperature information. There were pros and cons to the system. IIRC, I ran 285 BFGMT’s then at 60 psi. In the cold the tires would start off at approx. 53-54 psi, then increase in pressure as they warmed up.

Bill

Edit to replace picture.

[Grench]

Originally Posted by Nay

65 PSI

I have found that BFG's need more air pressure to handle well...usually 35 and above, but then they ride harsh over every little bump, but the MTR's? You must float all over the road at that PSI. Do they even touch the ground ?

When you go to a bigger tire with heavy sidewalls that gives you the option to run a reasonable PSI without sacrificing handling. My trxus are load range E and they ride better around town at 31 PSI than the 285 BFG AT's they replaced without handling degradation. They just soak up the big and small stuff so much better. I can't even imagine 65 PSI, but what works for you works for you .

Nay

Although harsh, it doesn't wander on paved roads. Washboards on gravel roads are a wholy other story. Washboards are downright scary at that inflation.

I think this is the extreme example of the hardest tire you can get away with on an OME 2.5" 850/863 lift. Fall is here, so it is time to drop them down a bit as traction becomes more important than cornering.

[-Spike-]

Originally Posted by Photoman

Don't know if this will help or not, but here is a product that I used called Smartire. The sensors mounted to the wheel and sent information to a display I had on the console. It gave tire pressure and temperature information. There were pros and cons to the system. IIRC, I ran 285 BFGMT’s then at 60 psi. In the cold the tires would start off at approx. 53-54 psi, then increase in pressure as they warmed up.

Bill

Edit to replace picture.

That looks pretty sweet! I imagine changing the batts would be a 'con'. Any others you remember?

-Spike

[Photoman]

Originally posted by Spike
That looks pretty sweet! I imagine changing the batts would be a 'con'. Any others you remember?

-Spike

Some cons.

The batteries in the tire senders were sealed so had to be sent back to the company for replacement. They were supposed to last six or seven years.

Each tire sender had its own frequency or way to identify itself. If I rotated the tires I had to make notes where each tire was so I would know the tire location of which pressure and temperature the receiver was showing. Each wheel had to be marked with a number to identify it.

In order to preserve the battery in the tire sending units, the units shut down when not moving. So until the wheels turned several times, there was no pressure or temperature reading.

The tire sender units came with a counterweight that offset the weight of the senders. It made it more difficult to balance the tires depending on where the sender units ended up being placedin the wheel.

Care had to be taken when mounting/dismounting the tires as the sender units stick out from the wheels and the tire easily would catch on them, possibly ruining the sender.

Money. IIRC, I paid $600 or $700 for the RV (recreational vehicle) package to get six sending units because I carried two spare tires and wheels.

I have no idea how much these things are now or if changes have been made to them. It was nice to have a warning light and audible signal if a tire was going low or getting hot and to monitor the tire temperature and pressures without having to physically check them. These things would also be great for pulling a trailer as the trailer tires could give a warning and/or be monitored from the towing vehicle. HTH

Bill

[mot]

cary,

As if lights and oil weren't enough, you need to pick another fetish?