I'm with you Josh37 My truck was off by 10mph at 65mh Drove me crazy. Put on bigger tires, now its only off by 5mph. Not right, but biggest tire that would fit my wheels. Your saying your speedo is off 40kph at 200? That's 20% Is the 20% consistent at other speeds? Not familiar with the product, but there are items like this http://www.bluegauges.com/prod_yellow_box.htm online that claim to be able to plug in & make the corrections. If your error rate varies, then you probably need a new unit.

It may be a little thing, but some of us are just too CDO. That like OCD but the letters are in order like they should be. :-/