Here are some industry resources that may be of help
State of the Industry Annual Report (SOI) published by the National Association of Convenience Stores
US Energy Information Administration
Check with your state agency that oversees the Monthly Motor Vehicle Fuel Distribution Report and Motor Vehicle Fuel Forecast(s)
Professional Car Washing and Detailing Survey
You also need to find out what state agency(ies) regulate gas stations within your state as there will be market wide and potentially local area market data. In addition the state dept of revenue can report gas tax collection data.