The first step is to find the chart that goes with your stock tires. Locate the pressure that relates to the recommended pressure on the tire inflation placard. Then, look for the weight the tire should bear at that level of pressure. Next, get the chart for your new tires. Look for the same weight there. If that specific weight isn’t listed, select the next highest weight and record the recommended pressure.
Let’s go through an example, using a 2005 F-150HD that originally had 245/70R x 17D Load Range D tires.
According to the tire placard, at maximum load, the tire pressure should be 50 psi in the front and 60 psi in the back.
When you locate that tire at those pressures on the load inflation chart, the rated loads are 2,205 lbs. and 2,469 lbs.
The proposed new tires are 285/70R x 17D Mickey Thompson MTX Load Range D tires. The closest numbers on the load inflation chart for those tires is 2,340 lbs. and 2,540 lbs. These equate to 40 psi and 45 psi, respectively.
If you need to adjust for differences in the pressure or weight between the charts, you can do it using this formula:
Tire weight/tire pressure = Load capacity pounds per psi
You would start with the load and pressure closest to the original tire, adding or subtracting to find the proper adjusted pressure. In this example, the calculation would be:
Original tire: 2,205/50 = 44.1 pounds per psi
New tire: 2,130/35 = 60.8 pounds per psi
To determine how much tire pressure to add, subtract the weight of the new tire at pressure from the weight of the old tire at pressure. In our example, this results in 75, or 2,205 minus 2,130. Next, take that result and divide it by the pounds per psi of the new tire. This equates to 1.23, or 75 divided 60.8. Adding the 1.23 psi to our front tires would amount to 36.23 psi, which we rounded up to 37.
For the rear tires, the old tire had a slightly lower capacity than the new one. In this case, you could go with the 45 psi or let out some air to reach 43.6 psi.
Let the chalk tell you
You can also “calculate” your tire pressure with the chalk method. This involves coloring a section of your tire with chalk to see how much tread is making contact with the ground. Start by finding a flat road surface. Concrete is actually the best choice, but you can also do this on asphalt. Make a mark with soft chalk that goes all the way across your tread. Then, gradually drive your truck forward about 50 feet and then backwards 50 feet.
Analyze the chalk on the tire. If the chalk is only worn off on the center of the tire, reduce the tire pressure slightly and go through the process again. With the adjustment, you should see the chalk wear off more broadly. Keep making tiny adjustments in the tire pressure until the chalk wears off evenly and all the way across the tread.
You will have to complete this process for each of your four tires. Once you’ve found the right street pressure, add 10% to all four tires. Then, measure the tires and add pressure to balance them. As explained above, you need to measure from the wheel to the ground. Start by balancing the profiles of the front tires with each other. Then, balance the front tires again with the rear tires. Always adjust the tires with the smaller profiles by adding air.
The advantages and disadvantages of this method are the same as those involved in the second method above. If you go through the process correctly, you end up with the ideal tire pressure. But, this method is tedious and there’s a reasonable chance that you will make a mistake.
Testing your tire pressure
You should always test out the results of your tire pressure calculations with a heat test. If tires are under-inflated, unnecessary friction and rolling resistance will result. Those conditions generate heat. Manufacturers use tread pyrometers to test tire heat, but this can also be done by measuring the change in tire pressure. Note that this test is mainly for preventing under-inflation.
Do this test on a day that’s warm, but not hot. Ideally, the outside temperature should be mid-70s to low-80s. To set up the test, locate a relatively straight section of highway that’s 13 or 14 miles long. You want to drive at 55 mph, or the maximum speed limit, for about 10 or 15 minutes. The tires should be at resting temperature, so you may need to stop and relax in the shade somewhere to let the tires cool before you begin.
The first step is to measure the tire pressure of your cool tires. Then, hop on the highway and drive at the maximum speed limit for at least 10 minutes. Pull over safely and take another reading of your tire pressure. If the tire pressure has risen less than 10 percent, then you are done. Your tire pressure passed the heat test.
If the tire pressure has risen more than 10 percent, there is more work to be done. This is because a rise in tire pressure indicates a corresponding rise in temperature. For every 10 degrees Fahrenheit, the pressure will rise about 1 psi. In other words, a 10-psi increase equates to a 100-degree temperature change.
A compressor will come in handy here, so you can add air pressure immediately and run your test again. Start by adding a few pounds of pressure to each tire that demonstrated a large pressure change. Then, let the tires cool and take another 10-minute drive.
Note that tires with aggressive tread patterns tend to run somewhat hotter at the ideal pressure relative to street tires. This conclusion is based on observation and experience only, and there’s no conversion formula to quantify this difference. Just make a mental note that if you are driving on mud-terrain tires, they may generate more heat than a tire with a milder tread pattern would.
Once you complete this test, you should also drive the vehicle and pay attention to the handling, particularly around corners. Let your subjective judgment tell you whether the tire pressure is appropriate or not.
Adjusting for loads
You can use the methods described above for any load state. Just remember that if you add or remove weight from your vehicle after testing, the tire pressure will need to be retested as well. You could complete these tests again every time you change the load, but this will obviously be time-consuming. Many truck owners prefer a less exact approach. They will inflate the tires to max load when the rig is fully loaded, and then let out air for unloaded driving.
Pressure changes over time
Most tires drop about 1 psi of pressure monthly. Changing seasons will also reduce tire pressure, usually at the rate of 1 psi per 10 degrees of temperature change. For example, a tire that reads 31 psi at 30 degrees might read 35 psi at 70 degrees. The only way to address these changes is to check your tire pressure often and at regular intervals. You will be rewarded for your efforts with better fuel economy and more even tread wear.
One last note
The last three methods described above require patience and attention to detail. If you can’t dedicate your focus to the process, then use the max pressure noted on the sidewall.
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In short, the PSI of your tire depends on the weight of your vehicle and also in many cases how you plan on using your truck.
Here's the PSI chart I was referring to:
http://cache.toyotires.com/sites/default/files/imce/LoadInflation_Table_P-LT_102913.pdf
And here's some nice reading on tire load rating:
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=55
