I have two wireless Cateye computers (CD300 & new Micro wireless) plus a third other brand on my third bike. They all agree within a tenth of a mile when taken on the same course. In each instance (and they have three different sized tires 700x19, 700x23 & 700x25) they were simply calibrated using the scale given in the instruction books. Over or under inflation will certainly affect the accuracy. The most accurate way is to mark the bottom of the tire, place that mark on another mark on the road, and with your full weight on the bike, and the tire inflated to what you run it at, measure the distance travelled by the bike in millimeters, and use that as your calibration figure. Some simply roll the bike - that is insufficient. You must have the weight of the rider in the matrix.