The methods used to develop commercial sales forecasts are multiple and vary from simple analogy to very complex spatial interaction models.
The analog-based or ?rule-of-thumb? procedure you described (gas/wash rate) is relatively inexpensive to use. However, this simple rule-based method is over simplistic and subjective and the results are often grossly wrong.
Common experience has shown that gas/wash rates vary from 25 to 300 gallons. The rate is affected by several factors.
Market ? Demography (pop, HH expenditures, etc.), traffic, proximity to competitors, etc.
Convenience ? Location, visibility, access, parcel size and configuration, etc.
Image ? Brand, visual appeal, customer service
Value ? Type of carwash operation and quality
Moreover, like most
marketing relationships, gas/wash rates are S-shaped. In other words, the relationship between gasoline and carwash volumes is not linear.
Another problem with using the ?wag? method is production. If sales volumes are over-estimated, the site may never realize the expected return. Conversely, if sales are under-estimate, the site may never realize its potential.
I speak with a background of over 20-years of modeling and forecasting experience.
With this in mind, you may want to consider the formal and rational use of data, rather than a wag, for something as important as a capital investment.