The V8 Owners Forum -

If you change the rear axle ratio or wheel size etc, your speedo may not read correctly. If an electronic unit, Jaycar used to make an adjuster which changed the pulse rate to what was needed, but it appears to be NLA. It was also a kit which required building.

This ready made unit is now available:-

http://efi.ttrignition.com/speedo-corrector.html