Okay that makes sense. I figured it was some sort of standard ratio. My guess wasn't too far off, looks like the ratio is 16.7:1

I wonder if all speedometers are the same? I'm working on a conversion that uses some GM parts and it appears that the speedo ratio that GM used is the same as the Mopar one. The speedo gear chart for a GM transmission is the same ratio as Mopar chart. The numbers are different since the number of teeth on the shaft are different but the ratios are the same. The Richmond transmission I'm using has a 8 tooth gear on the main shaft while a 727 transmission has a 13 tooth gear, but the ratio between the driveshaft speed and the speedometer cable speed is the same for both. Which leads me to believe that GM also used the 1000 RPM cable speed for 60 mph.