I stated it above. What he wants you to do is use a GPS. When you are going 40mph on the GPS write down your indicated speed on your speedo. Then do the same at 50mph & also 60mph. Give him the #'s & he can let you know if it's a common ratio that he has in stock. Or if it isn't quite as common (like mine) he will build 1 for you. It's an easy fix for $80 or less. I'm really enjoying not thinking about how fast I'm actually going while driving now.