Quote Originally Posted by junkyardjeff View Post
Late 70s C-4
In that case, Ford, and the aftermarket do not offer a 23 tooth speedometer gear

These are your options for the C4 automatic transmission.
19 tooth-tan
20 tooth-orange
21 tooth-red
23 tooth-white

At this point, you are going to have to deal with tire size changes (small, simple, yet can be costly).

Another option is to find a measured mile (a real one, not what you see marked on the side of a typical road), take measurements, then send the speedometer itself out for re-calibration to match what you need it to be.

Looking back, you did not make mention of whether you are using an original or aftermarket speedometer. Which are you using, be as detailed as you can be, include tire sizes (rear only), rear gear ratio, type of rear end, speedometer head, any adapters, etc.