Steve,
I assume you have a mechanical Autometer Phantom speedo, so it isn't easily calibrated like the electronic version is. I think the only mechanical solution is a gear-reduction box that goes between your speedo and the cable. Call Autometer for the source of these units.
I know this isn't the simple solution you're looking for but I wonder how much Autometer would charge you to install electronic workings in your speedo? Then you'd have a couple of different options for driving it (VSS, GPS) and calibration would be easy.
Here's another electronic/GPS option although not an inexpensive one:
http://www.speedhut.com/ecomme...rive-Speed-Converter . Dakota Digital has a similar item:
http://www.dakotadigital.com/i...mode=prod/prd886.htm
