After the fact, it depends on capital cost of improvement.
For example, if capital is $3,000 per month (low side) and gross net is $6.00 per car, that’s 500 more washes a month to break even on improvement.
Another method to consider is solving with waiting line theory.
Either approach requires information such as car counts, take rates, prices, cycle times, traffic count, demography, cost of goods, etc.