I was more commenting on the air pressure affecting weight capacity, which applies equally to oversized tires as well as just e and d rated tires. You kind of made my point for me actually, if the OP runs 12.5" wide tires on a 7" rim (assuming he can't find a shop to mount that setup, I didn't even notice he put up a tire size) he's going to have to air down the tires pretty far to account for the bulging tread block. So if he airs down from 55psi to 30 psi... his D-rated tires are barely going to carry the weight of his truck. If he drops an e-rated tire from 75psi to 55psi (which is what I do to flatten out my rear tires) he still has the full d-rated capacity. Google "load and inflation tables" and you'll get exactly what I'm talking about.

When it comes down to it, a D-rated tire in the OP's specific application may not be rated for the weight.

 

I'm not sugggesting what I done is right or should even be done. I'm just telling the OP what I done and how it turned out for me.
I bought my tires at wal-mart. Libarators 285 75 R16. I bought the services pack and warranty and have had them rotated every 6k miles. These are D rated tires and I have 100,004 miles on them now. Yes it been time for new tires. I'm in the threat checks now. I haul heavy, I haul often.
This set of tires has worked for me. It is what I will prolly go back with.

 

What D rated tires are you looking at that only have max of 55 psi? My coopers and many others I have looked at are rated 65 psi.

 

In that aspect, would I be able to run lower PSI when empty and run a higher PSI when I haul something? The stock wheels are just to get me started, I'm hoping my wallet will allow for wider wheels. As far as a shop goes, I have my own air powered tire machine, so I can do what ever when ever. How wide of a wheel should I be looking at? 10"?

 

You need to look at the specifications for the tire. Tirerack.com is a good source for info like that on a lot of different brands, all in one place. But each tire manufacturer should have the info on their site as well.

You have not mentioned what tire you are considering.

But for example, a BFG TA KO 315/70-17D is designed for 8.5-10" wheels. Tread width is 10.5" and diameter is 34.5". Your stock wheels are 7.5". Your wheel is only 1" less than recommended, and lots of guys run them without complaint. It isn't ideal, but nothing about this tire/wheel combo is ideal IMO. Judge for yourself.

Yes. That's ideal actually. Just don't forget to do it.

 

X2 to everything Bill said. I'll throw in some of my personal experiences to show what I mean better.

- My factory 265/70R17 BFGs that came on the truck were a ~10.5" wide tire on a 7.5" rim for a 3" difference, and I ran them at a full 75psi all the time no matter what. No bad tread wear, the ride seemed fine from what I remember, and load factor was never a question.

- 295/70R18s on factory 8" rims, they're a hair over 11.75" wide, so a difference of almost 4 inches. In the back there isn't enough weight to flatten then out, so I run 50-55psi and that's usually almost enough. Up front with the diesel and Ranch Hand, I run the full 80psi, mainly because I get squishy steering if they're underinflated, and they ride almost flat at the pressure. Add on a 7k trailer with ~800# of tongue weight, and the rear tires took 70 psi, which probably kept the weight rating up high enough on the inflation table to match that axle load.

- 35x12.50s on 9" wide aftermarket rims, difference of 3.5 inches, but I couldn't get them flat for the life of me. Sidewall press was 55psi IIRC, and I put them as low as 25 but still wasn't close on the back. I don't know if that's just an issue with ProComp Xtreme A/Ts, or if other guys have that issue, but I couldn't ever get them close. Never towed with them so I never messed with changing the air pressure.


I have a TSI tire inflator, it's got an electric gauge and a deflate valve plus a 4' foot hose so I'm not staring the bead in the face to change the air pressure. It's been within a couple % compared to every cheapo electric gauge I've tried, makes life a lot easier if you're going to be playing with your air pressure a lot.
TSI Digital Tire Inflator, Model# DG-20 | Tire Repair + Sealant | Northern Tool + Equipment

 

One company's tire is not the same as the next....
Reference point: I run 315/75-16 tires.
Previous tire: BFG A/T K0 load range D.
Current tire: Goodyear DuraTrac load range E.
The BFG is a pretty stiff D tire, the GY a reasonably flexy E.

The ride quality at a given psi is almost identical. Sidewall flex has not been enough of an issue to notice with either tire.

Biggest load: On the BFG's with super tall sidewalls, two quads on top of the bed rails, bed full of firewood, bumper pull trailer behind. Load on rear axle was right at the limit of the BFG load rating at its maximum 50 psi. If that isn't the worst case scenario (short of overloading!), I don't know what is. Still handled and drove very, very well. No complaints and would do it again.

I went to the GY's this time to have more reserve capacity and the BFG does not come in an E rated version in this size.