It has come to my attention that my speedo registers about 10% slow, i.e. at registered 66 mph, the car is actually going 60 mph...anyone know why this is the case?

 

Have you changed your diff or tires to a size that is non-stock? I found that my tires, from one brand to another changed my speedo by over 5%, and I changed my diff, so the other 5% was expected... maybe the brand of tires you use have a smaller sidewall than stock? Or you're running aftermarket wheels?

The Beast is back and badder than ever!

 

I do not know about the different colored pins. I bought this car about 4 months ago, and I think it has the original differential. It definately has the correct tire size...but has 14x7 wheels, which would reduce the overall diamter slightly...would it do it that much?

 

If your tire is narrower, then it will affect the overall circumference of the tire/wheel. You can correct this by finding the correct color drive gear for your speedo like Amber mentioned... so if you're running say 185-60-14 rather than 205-60-14, then it doe make a difference, just an example.

The Beast is back and badder than ever!