$ \text{Fuel Economy (MPG)} = \frac{\text{Distance (miles)}}{\text{Fuel Used (gallons)}} $
$ \text{Total Cost} = \text{Fuel Used (gallons)} \times \text{Price per Gallon} $
$ \text{Cost per Mile} = \frac{\text{Total Cost}}{\text{Distance (miles)}} \quad \text{or} \quad \frac{\text{Price per Gallon}}{\text{MPG}} $
$ \text{Fuel Economy (L/100km)} = \frac{\text{Fuel Used (litres)} \times 100}{\text{Distance (km)}} $
$ \text{Total Cost} = \text{Fuel Used (litres)} \times \text{Price per Litre} $
$ \text{Cost per km} = \frac{\text{Total Cost}}{\text{Distance (km)}} \quad \text{or} \quad \frac{\text{Fuel Economy (L/100km)}}{100} \times \text{Price per Litre} $