To get an accurate estimate, you should:
* Specify the model year: Fuel economy standards and engine technology changed over the years.
* Check the EPA's fuel economy website: The EPA (Environmental Protection Agency) in the US, or the equivalent agency in your country, provides official fuel economy estimates for vehicles. Search their website using the year and model of the Ford Focus.
* Consult your owner's manual: Your car's owner's manual will list the expected fuel economy.
Without more specific information, I cannot give you a reliable miles-per-gallon (mpg) figure.