Good info here. Now I gotta go try mine and compare it to a radar speed limit sign. Which isn't exactly a perfect science, I get it, but it's pretty close!
Edit: I went out and set mine to 7% and that fixed my Scangauge almost perfectly. I already knew I was off by about 2 MPH because I used several certified radar devices to check it. I had already known about the Speed adjustment but didn't know whether to go up or down and just feel like experimenting. I should've experimented though because it was a snap.
I'm talking about how I changed the Scangauge in my truck, not my Civic and the tire change was from 205/70/16 to 255/55/18, wherein I measured that I gained about two and quarter inches.