Correcting Speedo after Wheel change.

Is there an easy answer to this question I have always overlooked somehow?

I went from the factory 13" (original tire size unknown) wheels to a 15x7 wheel with a 225/60.

The car is a 225/auto 66' Dart. I know a speedometer gear is the fix, but how do I go about locating the correct retrofit? Please advise. TYVM