I checked speed using gps and found 2 mph difference, is this a considerable drift that will require calibration ?If the speedometer is not correct, that affects several things. Timing of shifts, fuel mileage being the main 2. My '16 with 4" lift and 35x12.50 tires dropped from 24-26 to 19. Recalibrated speedo and it came back to 20-22. Use a GPS speedo on phone to check speedo accuracy.