Most drivers have no idea what their car really or 'actually' burns. Fuel consumption measured over real driving is the number that connects your fuel bill, your driving habits and your vehicle's condition.
Two ways to express the same thing
L/100km says how much fuel for a fixed distance; mpg says how much distance for fixed fuel. Europe and much of the world use the former, the US the latter. They're reciprocals in spirit, and understanding both keeps comparisons across countries honest.
Getting an accurate measurement
The reliable method is to fill the tank, reset the trip odometer, drive normally, then fill again and divide the litres by the kilometres. Doing this across several tanks averages out the variation from traffic and conditions.
What shifts the number
Speed, acceleration, load, tyre pressure, weather and whether the engine warms up all change consumption. Knowing your real figure makes it easy to see the benefit of smoother driving or to spot a problem like a clogged filter or underinflated tyres.
Frequently asked questions
Divide the fuel used by the distance driven. If you used 40 L over 500 km, consumption is 40 ÷ 500 × 100 = 8 L/100km.
L/100km is fuel per distance (lower is better); mpg is distance per fuel (higher is better). They're inverted views of the same fuel economy.
Use the formula mpg = 235.2 ÷ L/100km. So 8 L/100km is about 29.4 mpg. This calculator shows both.