Hello Ostin77,
For your 03 GMC Sierra 2500HD 6.6L, the design of this Speedometer Calibrator is to let your vehicle know Tire size and new gear ratio, no shift points info from device. You can update this speedometer calibrator HYP732500 to the Max Energy calibrator with Hypertech via website (I do not know how much they will charge to upgrade your calibrator). We do offer the Max Energy Power Programmer HYP32500 which will update shift points (does speedometer calibrator function plus more features). Thanks for checking us out.