You'll find a much more accurate answer by:
* Checking your owner's manual: This will give you the EPA estimated MPG for your specific Mustang.
* Looking up fuel economy data online: Websites like fueleconomy.gov (US) provide fuel economy ratings for different models.
* Calculating based on your own experience: Track your fuel consumption over a few tanks to determine your actual mileage.
Expect a range anywhere from roughly 300 miles to over 500 miles on a full tank, depending on the factors mentioned above.