Speed Meter Contoller setup info required.

[se7en]

I have just bought a 96 Surf today and I have noticed it has got a speed meter controller fitted with (SMC 88 house LTD) printed on it. It is to enable the re-callibration of the speedo when fitted with non standard tyre sizes, unfortunately the the speedo appears to be reading incorrectly as when showing 80 mph the actuall speed is only about 70mph, and I don't have any set up information on how to adjust the callibration to make it read accurately.
I have carried out an intensive web search and found Zilch I wondered if any of you on this forum could help with any information.

Many thanks