Dually air psi question

A dually never needs 80 lbs inflation in LT235/80R17E's at rated at 2470 each in dual applications at the factory 65 lbs placarding. That is 9880 which is over the rating of the current dualies RGAWR. Over inflation increases the chance of impact damage to a tire and decreases the contact patch and braking.

But old habits die hard!

At the 40 lbs a lot of people run empty they are rated at 1725 each or 6900 total, which is good for throwing around 3K in the bed for casual hauling.

If you can produce the weigts on tires, and give tire specifications, I can calculate a safe pressure for you. Because of the 75/65 on car I assume OEM tires to be E load and not P-tires.

Depends how much weight is on the back of the truck.
Anywhere from 30psi empty up to 80psi fully loaded down.

I just run 80 psi all around all of the time on my dually. It's nowhere near a teeth shattering ride and pretty close to the ride of our old 1/2 ton. Nothing like riding in old duallys. My wife uses it as her daily driver often and doesn't have any complaints. The only thing I've noticed is that sometimes the coconuts in the passenger seat go a bit crazy on rough roads, but I'm in no way complaining about that:B

I have no clue. My repair shop folks take care of my tires when I get my oil changes. I never bother checking them. They seem to work and handle the truck just fine, towing or not. Tire tread wear seems completely even on all tires too. So, I don't worry about it.

I think if you are somewhere in between the two PSI recommendations you're good.

Now the trailer, my Carlisle tires have 80 psi on the side wall. I have them aired up to 80 psi and tow at 60 mph or less.

The tire pressure specified on the vehicle tire placard (the sticker on the doorframe) includes the max load and max towing (as specified by the vehicle manufacturer) for the tire sizes listed there. If you are towing within those limits, then that pressure is appropriate.

ive been running the ford recommended 65 rear/75 front and have yet to see an issue.

Running 80 lbs on the rear tires of a dually will cause your wife to file for divorce and your teeth to get loose. Tire pressures are like speed limits. They are a limit, not a minimum.
You now have 4 tires doing the work of your former 2. Those 4 new tires have a weight limit of about 12K lbs at 80 psi. Try the 65 for a while, I think you will be satisfied.

I run mine based on recommended tire pressure on the tires themselves. When towing I run max pressure on rear and about 5 lbs under recommended max on front.