From the
AndyMark web page describing the Supershifter:
STANDARD GEARING (this is what is shipped with a Super Shifter order)
Ratio between High and Low gears: 2.56:1
FINAL STAGE, w/ 45:20 GEARS:
Low Gear: 24.0:1
High Gear: 9.4:1
so just divide motor rpm by the gear ratio for low and for high and you'll get your answer. Of course, if you're using a different AM transmission you'll need to get the numbers for that one.