Speedos are usually optimistic. Odo's however, are usually pretty close to accurate, even though they share a PTO. I was led to believe this was for legal reasons.
Cant have a speedo read under your actual speed, that would lead to developing relationships with policeman friends.
And inaccurate Odo's lead to issues of mileage at resale, salvage, etc...
My speedo is about 10% off... way more noticeable the faster you go, as evidenced by GPS. Mileage is much closer. My girlfriends bike and mine are only off each other by 1 or 2%, 3 max. I noticed the same sort of thing when riding with a bunch of other guys when we all have roll charts. Everybody is off by a couple of tenths at the resets.