In Reply to: Y2K owners - is your speedometer accurate? posted by Tired of getting passed by Escorts! on January 15, 2000 at 16:50:51:
Mine, like many (most? All?) reads around 8% high.
The OBC should be accurate. I don't have one on my Z3, but the one on my 328 would be perfectly accurate (near as I could measure) when reset in the speed mode. The OBC and odometer use the same measurement from what I'm told.