This might help you figure a real close to what you got for rear gears.
Have you verified the tach for accuracy with a digital timing light? You can determine if the speedo is correct/close also with this chart. With +/- involved Special K is pretty close. Your tire OD should be 28.7" in a perfect world so bump it to 29 on the chart
https://auto.howstuffworks.com/auto-...size-chart.htm