I bought my truck used, it came with a 4" turbo back exhaust already. Althought I can't find any evidence of a tune, I'm assuming the SCT will over-wright any tune that may already be flashed. So it really dosen't matter wheather it already has a tune or not. From what I can tell, there isn't actually a chip being replaced. So when you tune it, your really just flashing over whats already there, effectively erasing a tune that was there previous. Is this a correct assumption?