I just asked this question like a month ago.

http://vehicross.info/forums/showthread.php?t=13532

I think mine comes out to be about 7% off. So, at 60 on my speedo, I am most likely going about 65. I need to set my scanguage up to calculate this, just haven't had the time to dig out the instructions. If you figure it out, post it up if you can....

Bart