I'm curious, how did you get water usage data from other car washes?
Water usage is merely a guide, not a predictor, be careful relying on it too much. This "calculating the revenue" is notoriously difficult.
There are lots of reasons why 2 washes that appear to be similar could have widely different water usage, such as minutes sold, efficiency of equipment (regens, softeners, etc), tips, pressures, hoses, customer/demographic washing habits, cleaning routines, recycling, weep, leaks, seasonal differences, landscape watering, etc etc. IBAs bring in another whole set of variables but you posted in the self serve section so we're assuming thats not relevant.