I have a 66 with C-4 automatic when going down the road at what the speedometer says is 60 MPH the radar gun says I am going 50. The gear that is in there now is a 16 tooth gear. Would I need more teeth or less teeth to get the correct speed to display?

 

You need to add teeth to slow speedo down. I'd say put an 18t gear, but that would be a guess. If you want to figure it out by the numbers go to the NPD Mustang catalog on the bottom of pg 139 there's a nice formula for the correct size gear. You need to know your tire size.

 

We happen to have an ole school shop in town that specialized in speedos. His rule of thumb was 10% speed for every one tooth, in this case the 18T sounds about right.