I have the 18" OEM Michelin 265/60 18

I tow 2 boats one is 5000 lbs loaded and fueled.

Without and TOW or items in BED, what pressure would you suggest?

The tire says like 44, and the DOOR STicker says 35 all around, and the all 4 tires are at 40psi...

 

The dealers often over inflate them on delivery. I think this is to bump the mileage slightly, but that is anecdotal. I've been running mine at about 36.

 

The number on the sidewall of the tire is MAX inflation pressure. so on an unloaded truck, I'd run a decent bit below that. However, if they are at 40 right now, and the ride is OK, you could leave it, it won't hurt anything.

 

Believe it or not, they've thought about all that already. I used to think it was required to bump the pressure up to the maximum sidewall numbers when towing or carrying heavy loads. See what your owner/operator manual has to say, tire manual, do the math etc. You might be surprised. It's very difficult to break old habits.

 

My opinion is that you want the tire to touch flat across the tread. But the pressure required to do that differs based on load, with more pressure needed for more load.

I have a concrete driveway that isn't used much and the concrete chalks. Drive over it and the tires pick up the chalk, showing how much of the tread is touching. That's the way I fine tune pressures - within the range given in the manual and less than the max on the tire.

 

Tire pressure is a factor in the load capacity of your tire. The load your tire can support is based upon being inflated to the max pressure on the sidewall. But often this capacity is much higher than the rest of the truck. The drawback of keeping your tire at max capacity is the ride quality isn't as good, there is less grip and the tire will wear unevenly. The tire pressure on the door sticker alleviates those problems. I can't seem to find an online calculator tonight, but past experience has shown me that the tire pressure on the door sticker drops the tire capacity to closer (but not under) your axle limits.

I didn't like the squishy feel of my 18" dia tires at 35 PSI, so I bumped it to 40 (42 actually according to the computer). My contact patch is still solid and the ride is better, so that's where I'm going to keep it. I suggest just finding a pressure that works best for you; but don't worry about your loads. You're still within the limits of the tire.