Guys, if you're recalibrating your speedo, don't forget that it may be possible to use that kit from Jaycar, it's a plug-in on-the-fly-adjustable calibration tool for electronic speedos.
It's been mentioned in these forums before, can't remember where, I'll find the thing on Jaycar's site again:
here is a direct link to the part in their Kits - Automotive section.