Tony,
Ford says that each tooth on a driven gear will equate to about a 4 MPH change. You can be much closer depending on your setup but may be off at some ranges. If it were me, I would go with one that reads just a little fast, IE maybe 1 MPH if I could to avoid the speeding tickets for driving 2 MPH over the limit. And yes, they do give them out for that, at least here. I got one going to work for doing 47 in a 45 MPH zone at 3 AM with no traffic except me and the cop. My speedometer was reading 45 but it still cost me $55 dollars.
Ron
