Since I changed tire from the stock 265's and went to 315's I followed my dad at 55 to see how far my speedo was off. Looks like running 55 my speedo says 50. I double checked it with a tom tom gps and sure enough when it said I was going 55 my speedo said 50. I hooked up my Cortex and changed the tire height to 35" and rechecked it. Still 5 mph off. I went from 33" to 37" with no change at all. No matter what I set it for my speedo still says 50 when I'm running 55.