I run 30x9.5x15" on my Tracker.  All I have is spring spacers, stock front struts, and Pro-Comp rear shocks.

Nice ride, dude.  

Ok, so, I mean if you get bigger tires, just slightly, I just want to get one or two sizes bigger, my speedo will change, BUT it won't hurt the truck in anyway right?  

Nope, Bigger tires will do nothing else except for the speedo.  You just learn what the speeds are.

Ok, that's a relief.  Thanks man.

.....and you wont be able to burn out as easily.

I have an "03 Tracker what's the largest tire theat will fit w/o mods?

It does change your speedo, but you can buy electronic modules that compensate for the difference.   I have seen several websites that have calculators to help you calculate the difference and you can just compensate while driving.  I don't remember the website, but it shouldn't be hard to find.

The general idea that, I have seen is that 235/75R15 is the biggest that you can fit without modification