[McGary911]

Interesting. I knew this was a glaring behavior on my E46, but also knew that the ODBII port output was the real speed. I did the same check on my M2 and thought, great, it's fixed. I guess not.

Validating the real speed is a lot trickier. I wouldn't trust those "courtesy" radar units. They're often portable, banged around and even the stationary ones have no good reason to be kept calibrated. You could have someone with a known accurate speedo pace you to verify, but how does that one become a known good? Same problem.

I think the most accurate way would be if you had a buddy in law enforcement to use his (radar) gun on you. They're not perfect but are periodically calibrated as their readings are often used in court.

I'm going to see what my handheld GPS tells me. That might be pretty accurate too.