There’s MPG as a method for measuring the fuel economy of a car, then there are methods that make practical sense. MPG itself isn’t inaccurate as a unit, but it doesn’t present an easily understandable method for the general public to use. L/100km is logical, it presents a linear unit to measure fuel consumption by. Where MPG fails is that it’s a measure of mileage, not a measure of fuel consumption: it’s an inversely proportional measure. MPG falls (in a non-linear fashion) as fuel economy improves, see graph below.
For example, when people see a L/100km rating, it’s easy to understand. An automobile with a 15L/100km consumption takes 1.5 times (150%) as much fuel to travel the same distance, than a vehicle with 10L/100km, what could be simpler? However, when somebody sees a car with 1.5 times higher MPG, it doesn’t mean it takes 150% more fuel, it actually takes 133% more fuel. This is because the amount of miles travelled (1.5x) is 33% higher than the original MPG. A far more sensible standard would be the inverse, a GPM measure (gallons per miles). This would then allow people to figure out the amount of fuel used, to travel a set distance.
Another example where problems occur because the scale is not linear. In a metric system, a set number, say 2L/100km, improvement in fuel economy is the same whether the car is small or big, anywhere on the scale. With MPG, a 5MPG improvement at the small car end (47 to 52) is equivalent to going from 5L/100km to 4.52L/100km, less than 0.5L/100km difference. A 5MPG improvement at the big car end (17 to 22) is equivalent to going from 13.8L/100km to 10.69L/100km, more than 3L/100km difference. Therefore, it’s much easier to claim big MPG improvements with small cars than big cars.
This causes confusion and misjudgment with the car-buying public. People like to compare things in scale with other things. Add onto that, that there are US MPGs, and there are UK MPGs, both of which are different, then you can see where the confusion occurs. Unfortunately, there is end in sight to this, however more people are adopting the GPM method of measurement, which yields similar results to L/100km.