On a 748/916/99* the speedo is driven by the front wheel so gearing changes don't affect the speedo. Most if not all monsters and most other Ducs do as well. Don't know where these new fangles Multistranglers or 999 types take the speedo feed from.
Using the front wheel pickup there will be slight differences between tyre makes and as the tyre wears due to differences in the circumference/rolling diameter.
Jap bikes tend to use a speedo drive from the gearbox so gearing changes do affect speed readings.
As for the over reading causing more services, quicker drop invalue etc isn' that why some helpful individuals offer "mileage correction"
Ray