Fuel = Distance / 100 x Consumptionper 100 mi. For example, if the distance is 300 miles and the consumption is 5 gallons per 100 miles, you will need 300 / 100 x 5 = 15 gallons of gas. Both formulas are applicable for metric units - km and liters, just make sure you do not mix metric units. Using an online fuel calculator will make sure you do