transmission attachment to correct speedometer

[Dick]

I have a Euro 230 SL with the original speedometer in km's.  I have installed a rear-end with a higher ratio.  Recently, I heard of a company (Bob's Garage) ? that has a gadget that attaches to your transmission to recalibrate your speedometer after you give him pertinent info.  This would give me the correct kilometers per hour which I am comfortable with.  Does anyone know anything about these attachments and the contact info.?

[jeffc280sl]

Is this what you're thinking about?

http://www.speedometershop.com/ratio.html