Dom,
I use a Dakota Digital speedo calibration unit. You dial in your speedo prefectly against your GPS no matter what combiantion you're using. Very simple to install and use. Change anytime you want. Mine rides in the center console for easy access. On ebay the go for about $75. Here's a link to their web site.
Hope all is well otherwise.
H
http://www.dakotadigital.com/index.c...rod/prd126.htm