flow meter

I would like to install a flow meter in my agriculture aircraft. The flow meter sets in a 2 inch housing inline of my spray system. As liquid flows through the housing it rotates an impeller which produces a 5 volt pulse for every revolution. 720 revs equels one gallon. I figured out how to display number of pulses on a seven segment display using a 555 timer and a LED driver. However I would like the circuit to display gallons instead. Currently, to get gallon I manually divide total pulses by 720.

Thanks for the help

AJ