Just bought a 2003 F-350 PSD. Has 35/13.5/18 Pro Comp tires. Never owned this big of tires. What pressure should I run in them.

 

The stock answer is to check the pressure label on the door jamb, but this label is assuming factory tires and a truck loaded to the GAWR’s (Gross Axle Weight Rating). If you are operating at a lower weight, you can reduce the pressure for better tire wear, smoother ride and better traction.

Here’s the proper way to determine what pressure to run in your tires. Weigh your truck, getting individual weights for each wheel (or at a minimum each axle). Then consult the tire inflation chart for your brand/model of tire. You can view a chart at www.trucktires.com . The chart will tell you what pressure to run based on your weight. If you got individual wheel weights, use the heaviest wheel on an axle to determine what pressure to run on all tires on that axle. If you weigh your truck unloaded you’ll need to do this again with your normal load. If your load fluctuates all the time, it’s probably easier to just keep the tires inflated to handle the highest load you anticipate.

In my case, I carry a 4000 lb slide-in camper occasionally. I keep the truck tires inflated for unloaded driving most of the time. Then before I load the camper, I pump them up for the extra weight. That way the truck rides smoothly with good traction and even tread wear while unloaded. If I was loading and unloading the camper daily or even weekly, I’d probably just keep the tires pumped up. Changing tire pressure daily on a dually would get old real quick.
It's a good idea to read the footnotes and fine print in tire inflation manuals also. For instance, in the Firestone manual it says that if you are driving for long periods at 65 to 74 mph while at the rated max load for the tire, you should increase the tire pressure 10 psi (provided the wheels are rated to handle the pressure). In the case of the Firestone Steeltex 265/75-16 tire that would be 90 psi. This means that the 80 psi listed on the sidewall is not the max pressure. If you want to drive 75 to 84 mph , you still need those extra 10 psi, but you also need to reduce you max weight capacity by 10%.

You can view a tire inflation table for many tires at www.trucktires.com .
As an example, here’s the inflation table for the LT265/75R16 Firestone Steeltex tire

Weight Pressure
1910 35
2100 40
2280 45
2470 50
2625 55
2790 60
3000 65
3105 70
3260 75
3415 80

65-74mph add 10psi
75-84mph add 10psi & decrease load 10%

 

There's also the chalk method. Find a FLAT, level surface to drive on. Then simply take a piece of chalk and run a straight line across the tread from one side to the other. Drive the truck until the chalk begins to wear. Then look at the wear pattern of the chalk. If it wore evenly then your tire pressure is fine. If the center of the chalk line wore more than the edges, the tire is over-inflated. If the chalk line wore more on the edges than the center, the tire is under-inflated. Do this with truck in it's normal everyday configuration while the tires are still "cold".

 

I have 37" procomp tires.50 psi in front,45 psi rear(unloaded).

 

I run mine at 80 psi. It makes for a rougher ride on the back roads but then it helps with the fuel mileage.