It wouldn't do anything to your speedometer reading if you use the correct size tires, as LT models come with 15 inch tires and 185/55-15 tires... as long as you're with the same size tires you should be fine... If you go wider, the height of the tirewall has to be shorter, but then you'll see the change in the speedometer reading.

I'm using 17" with 205/40-17 and some guys on here even have 18" wheels on theirs. The reading won't be off by much. There are multiple sites and tools to figure out the change in the reading, but it won't be anything major if you're smart about choosing your tire size...