MPG and L/100 km measure fuel economy differently
Miles per gallon tells you how far a vehicle travels per unit of fuel. Liters per 100 kilometers tells you how much fuel is used to travel a fixed distance. Because one is distance-per-fuel and the other is fuel-per-distance, the conversion is reciprocal rather than a simple multiplier.
US MPG conversion
A commonly used relationship for US gallons is approximately L/100 km = 235.214583 / MPG. To convert in the other direction, use MPG = 235.214583 / (L/100 km).
Example
A vehicle rated at 30 US MPG is approximately 7.84 L/100 km. Lower L/100 km means less fuel is used for the same distance, while higher MPG means more distance is traveled per gallon.
Check which gallon is being used
US and Imperial gallons are different sizes. Make sure the source specification says which MPG standard it uses before comparing vehicles.
Use the Fuel Economy Converter for US MPG, km/L and L/100 km.