Most new vehicles use a reluctor wheel with a magnetic pick-up coil and the Xterra is no exception. It could be recalibrated electrically with a simple reburn of the PCM, but therein lies the problem, no one cares to bother. The factory doesn't give a flying flip if our speedos don't read accurately after installing larger tires and the after-market cares even less about Nissan owners. I recommend doing what I did. Measure your differential with a GPS and then drive accordingly. Now if you owned a Chevy, Ford, Dodge or Toyota you could have an accurate speedo. But alas, we are lowly Nissan owners and have to sort things out on our own. Things are improving with each passing year, but the wheel of progress spins slowly when it comes to us. That is my way of saying, we have to deal with it until the after-market gods smile on us a little stronger.