Comparing vehicle fuel efficiency globally is complicated because the US uses Miles Per Gallon (MPG) while most of the world uses Liters per 100 Kilometers (L/100km). These metrics are mathematically inverted: MPG measures distance per unit of fuel, while L/100km measures fuel required per unit of distance.
To convert US MPG to L/100km, you divide 235.21 by the MPG value. For example, a car that gets 25 MPG consumes 235.21 / 25 = 9.4 L/100km. If you have UK Imperial Gallons (which are larger than US gallons), the conversion constant is 282.48 instead.
Calculating cost per distance
To calculate the actual fuel cost per mile or kilometer, you need both the efficiency metric and the local price of fuel. For a car in the US getting 25 MPG with fuel at $3.50 per gallon, the cost per mile is simply: Price per Gallon / MPG = $3.50 / 25 = $0.14 per mile.
In a metric system, if a car uses 8.0 L/100km and fuel costs €1.60 per liter, the calculation is: (Efficiency / 100) × Price per Liter. That is (8.0 / 100) × €1.60 = 0.08 × 1.60 = €0.128 per kilometer.
Bridging the gap for international travel
If an American is driving in Europe and wants to understand their cost in familiar terms, multiple conversions are required. First, €0.128 per km must be converted to miles by multiplying by 1.60934 (1 mile = 1.60934 km), resulting in €0.206 per mile.
Then, the currency must be converted. If the exchange rate is $1.10 per Euro, €0.206 per mile becomes $0.226 per mile. Only at this point can the American traveler accurately compare the European driving cost to their $0.14 per mile cost back home.