It would be out by 8 or 9%. I don't know where the speedo sender is either, but gearbox output is quite likely.
You can use these to recalibrate your speed if required
http://www.bikersdiscountstore.co.uk...r-v4-pd-18.php
However, as speedos show higher than the true figure and a larger tyre will cause a lowering of the reading, you might end up fairly close to the truth anyway!