Looks like you have an I/S, so all you need to do with the different wheel is install a SpeedoHealer. It's smaller than a pack of cigarettes and will fit just about anywhere, I've installed one for a friend and just laid it on the battery cover. You run the speedo pick up output to the Healer, and from the Healer to where the pick up originally went. To set the correction factor, just use a GPS to see what your actual speed is and what the indicated speed is. Go to the website and enter those numbers and it will tell you the numbers to punch in to correct it. I set my friend's about 3 mph high, he also has a heavy hand on his Valk.
Cost him about $110 five years ago, may be more now but well worth it for accuracy.
http://www.healtech-electronics.com/products/sh/
