To add to what Eddie just said....In reference to the comment about setting the programmer, ill elaborate. As Eddie said, the only TRUE measurement that matters is what your vehicles computer thinks your tire size is. This can be confusing also. If you put a larger than stock tire on, your computer needs recalibrated to adjust your speedo, shift points, mpg calculations, ect. When doing so, the computer is interested only in the distance from the center of your wheel/axle, to the ground. Thats due to the weight of the vehicle "squishi g" the bottom of the tire against the road. So when making that "tire diameter" measurement, it is most accurately done by measuring from center of wheel to the ground, (which gives you a radius). Then multiply that distance by 2 (giving you a diameter)