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/saf...ht Trucks.pdf#search="p-metric lt load range"
http://www.yokohamatire.com/pdf/tsb-070302.pdf#search="p-metric lt load range"
http://www.tirerack.com/tires/tiretech/techpage.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.
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/saf...ht Trucks.pdf#search="p-metric lt load range"
http://www.yokohamatire.com/pdf/tsb-070302.pdf#search="p-metric lt load range"
http://www.tirerack.com/tires/tiretech/techpage.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.
.
