Mine was off too, but remember that these speedometers are calibrated by a theoritical tire diameter and I found that these Conti's don't measure up to what the theoritical spec sheet says. Mine were 71.125" circumfrence and that is 22.63" tall, fronts are actually taller and that is totally against tire spec sheets.
Go to an interstate, run for 10 miles at 60 (not on a busy day) and see if it takes you 10 minutes. It's hard to hold a steady 60, but if you do you can see how far the speedo is off and then adjust Scan Gauge to match true speed.
And in the fine print, I'm sure they can wiggle out of the warranty...lol