Has anyone ever done a "chalk test" with their bigger tires to adjust the psi? I have 33x12.50R15 (on 15x8 rims) tires on my 83 and one day I drove over some loose dirt in the garage and the dirt only showed up in the center of the tread. Didn't put much thought into it but lately since I have been driving my truck alot I have noticed it drives like its on ice skates or something. If anyone of you watch NASCAR, the term "loose" describes my truck perfectly. Its a lot worse through turns and hitting bumps than it is going in a straight line.

I figure I need to lower the pressure in them (havn't checked to see what it is yet) and I was thinking about doing a chalk test to set the psi in them specifically for my truck. My question is If I draw a line on my tire how much should be left on the sides? I wouldn't think you would want full contact all the way across unless its off road or icy/snowy conditions. I'm mainly looking for normal use and gas mileage and I'm not to concerned about tire wear since its not my DD. Anyone have tips or pointers for this?

I also figure a high-steer kit from Marlin would help out with the driving. I do have a new pro-comp drag link installed that compensates for 3-4" of lift. It can be seen on the first page of my build.

 

I also found this thread on a Jeep forum on doing a chalk test and what this guy does to set his tire pressure:

"When I run a chalk test, I use a flat, empty parking lot. One with room to get up to city street speeds. Mark the tires, and do a slow figure 8.
This is to get into the balpark of how the edges are hitting.
Mark the tires again, and do a larger figure 8 at a slightly faster speed. check again and make adjustments as needed.

Then comes some slalom turns, then some "emergency" type swerves.

In a straight line, my tires have anywhere from 3/4" to 1 1/2" of chalk on the outside edges. after the figure 8s, slalom and swerves, its down to about 1/4" to 1/2" on them. I also check to see if the tires are getting unusually warm. Are the giving me good "feedback" while testing or are they getting squishy or mushy."

 

I have not heard of the chalk test, however, I was wondering if the rims are just a tad too narrow for the tires. I have lots of friends running this same setup, I just know that if the rims are too narrow, it can cause a little bit of a rounding of the tread, which in turn, causes the tires to run on the center. The way to fix this is to run the tires at a lower PSI. Also, since your tires are wider, there is less pressure on every square inch of tread touching the ground, which means that the tire pressure should be lowered even further in order to have the tires run evenly on the ground. I do not remember the standard pressure for the stock tires, but my guess would be that your tires would need to be at a lower pressure in order to compensate for the width of the tire. I am sorry I could not be more helpful, this again is just what I remember from my memory and for all I know, my information could be wrong.
All in all, I would just run them at the pressure you think is correct, and just look for uneven wear, if it is wearing on the center, lower the pressure, if the edges are looking pretty worn along with the sidewall, definitely time to air up.