I think I figured it out. My rear tires are 88 inches circumference so 60 mph is 720 revs/mile. With a 3.23 rear gear that makes the driveshaft speed 2326 rpm. To get to 1000 shaft speed for the speedo cable I need a 2.326:1 ratio gear reduction. The Richmond has a 8 tooth gear on the main shaft so I need 18.60 teeth on the speedo gear. GM offers both a 18 tooth and a 19 tooth gear so I'll start with the 19 tooth gear and see how it does. It should read 2% slow if my math is correct. I don't know how well the speedometer itself is calibrated. It is the original speedometer in my '65 Coronet so it might be a little wore out.