Mine is exactly 10% off, less than GPS.
It has differences on all the speed range, from 10 to above.
So, if gps is not mounted on the bike I do calculate it 10% plus speed.
These are my appoximate speedo speed limit for my bike to prevent ticket ha ha ha
My other street bike has same speed error too.
* 35 zone >> 40 (actual/gps speed: 36)
* 45 zone >> 50 (actual/gps speed: 45)
* 55 zone >> 60 (actual/gps speed: 54)
* 65 zone >> 70 to 75 on speedo (actual/gps speed: 63 to 67)
* other >> not tried yet... ^^; ha ha ha