Here's the deal, I know someone who has 285/75/R17 on the same truck that you drive and has no problems (clearance wise). I have since put 285/70R17 on my '99 Yukon with very little bump on full suspension travel. My speedo is off exactly 6.2%. I simply rely on the math to tell me how fast I am going. I have noticed however that I only calculate it when I see a cop. :biggrin: Now I have read and am pretty sure that the Hypertech Power Programmer will adjust the Speedo if you have changed tire size. You simply plug in your tire size and it automatically adjusts. The PP is about $350-$400 but in addition to an accurate speedo you get improved performance!