My truck still has the original spare tire, it is the Michelin XPC 4x4, the exact same tire that was on all four wheels from the factory.
It is a 6 ply ("C") tire rated at 2039lbs at 44psi, that is the max load and tire pressure that it can safely handle.
As long as your tires can handle the load they can be whatever ply you want.
The more plys (layers of rubber and steel) the more abuse they can handle.
A tire shop will not sell you a tire that cannot handle the weight of your car or truck because they dont want to be liable if your tire blows out because it was not strong enough.

Tire Tech Information - Load Range/Ply Rating Identification

AntiChrist is 100% correct, determining the correct tire pressure for your tires is alot of work and pretty precise, 99.8% of car owners dont want to do the work (me included) so they just go with a run of the mill generic "good enough for me" tire pressure.

The more weight you carry the more plys you need and the more air you need to keep the tires sidewall from squatting.

 

Tire Tech Information - Load Reduction of Euro- and P-Metric Tires on Light Trucks

 

So Spike what tires are you running?
AntiChrist on the Disco, what tires are you running?

I have the Bridgestone Dueler AP in a P235/70/16 104S code.

 

Cooper Discoverer S/T, 235/70R16.
2094lbs at 45PSI.
6 ply, so a "C" rated tire, I have them at 35PSI.

EDIT:To correct my goof.

 

On my wife's '97
Chaparral Commercial Traction 215/85R16 10ply


On my '95
Old tread pattern Cooper Discoverer STT 7.50R16 8ply


Goodyear G90 7.50R16 10ply


Unfortunately my G90's are completely worn out and the Coopers are about there. I need to replace them very soon.

 

Gee Tom, I didnt know we were supposed to include pictures.

http://ssl.delti.com/tyre-pictures/C...scovererST.jpg

These are going on this weekend.
235/70R16
1984lbs at 44PSI, also a 6 ply tire.

http://lh3.googleusercontent.com/pub...WvGwB-cD3IsUKQ

 

LOL Well, the three I mentioned aren't very common, and you can't get the Coopers anymore, much to my dismay. 7.50x16 is my favorite size for Series, Discovery 1's and Defenders.

 

I also like the old STT, I like the new ones better.
My steel rims had the 7.5/16 tire on them, with tubes!!
I wondered why they still held air with the weather cracks and dry rotting so bad it was ridiculous.
The tire shop scuffed up my paint job so I need to touch that up and then they get mounted up this weekend.
I like that Chaparral Commercial Traction tire, nice tread pattern.
Never heard of that brand before.

 

Yeah, I'm thinking of trying the new STT design, I just wish they still made them in 7.50R16, so I'm keeping my eyes open in the meantime for that size.

 

I really think you would like the Copper S/T's Tom, they come in a buttload of sizes and load ratings and last forever.