The Xcal is not as efficient for inputing tire size as the Edge is. The Edge goes by MM and you can input it to the tea. The Xcal goes by rev per mile and only lets you choose settings in increments of 5. Despite making changes on the Xcal I can't get my Edge dead on, it's still 1mph off from GPS.

 

I've got a Superchips 1715, I just went from a 265/70/17 to a 285/70/17 tire. My speedo was way off. My old tire was 31.7, new tire is 32.7 tire height. Superchips wants tire height to change speedo. So I thought how easy was that, changed it to 32.7, it was off even more than the stock setting. I used GPS to figure it out. First changed to 36, to see where it was at, then 30, 34,32, 31, 31.25, 30.75 and found that 31 works best. It ended up being off 1mph 0-59 and from 60-75 it is dead on. So, my suggestion would be go up a lot, go down and see which is closer, then you should be able to dial it in. I have to admit, this was a HUGE Pain in the (__!__). You would think they would make it easier for the user to do this. They should make it where you put in the tire size period! Good Luck guys...