I'm going to the 285/75 from 285/70, which it has already been calibrated for. Do I need to recalibrate for that extra 1/2"?

 

from Nittos web site there is a difference of 1.1 inched in diameter. Revolutions per mile are 614 instead of 634, that is a tad over 3% difference...so it depends how close you want to be. Do people re-calibrate to account for tire wear over the life of the tire? If you are staying with the Nittos...when it is time to change tires, then your speedo will be right on