Miles per gallon (MPG) measures how far a vehicle travels per unit of fuel — higher is better. Liters per 100 kilometers (L/100km) measures how much fuel a vehicle consumes per fixed distance — lower is better. The US uses MPG (with US gallons of 3.785 liters), while most of the rest of the world uses L/100km. The UK uses MPG but with imperial gallons (4.546 liters), so UK MPG figures are not directly comparable to US MPG.
The conversion formula between US MPG and L/100km: L/100km = 235.215 / MPG. For example, 30 MPG = 235.215 / 30 = 7.84 L/100km. Notice the inverse relationship — a small improvement in MPG at the low end (going from 10 to 15 MPG) saves more fuel than the same improvement at the high end (going from 30 to 35 MPG).
The non-linear relationship and its practical consequence
Because MPG and L/100km are reciprocals of each other, equal-step improvements in MPG are not equal in actual fuel saved. Going from 10 to 20 MPG saves 11.76 L/100km (from 23.52 to 11.76 L/100km) — halving fuel consumption. Going from 30 to 40 MPG saves only 1.96 L/100km (from 7.84 to 5.88 L/100km). The first 10-MPG improvement saves 6× more fuel than the second 10-MPG improvement.
This has real policy implications. Regulations that require all vehicles to improve fuel economy by a fixed MPG amount deliver dramatically more benefit to low-efficiency vehicles and barely affect high-efficiency ones. L/100km-based standards avoid this distortion because they're proportional to actual fuel use.
Real-world fuel economy vs rated economy
Official fuel economy ratings (EPA in the US, WLTP in Europe) are measured in laboratory conditions using standardized drive cycles. Real-world fuel economy typically runs 15–30% lower than rated values due to highway speeds, air conditioning, cold weather, cargo weight, and driving style. The US EPA's 5-cycle test tries to account for some of these factors but still overestimates real-world performance for many drivers.
Fuel cost per distance depends on both fuel economy and fuel price, which vary by country. A car getting 8 L/100km in France (fuel at €1.70/L) costs €0.136/km. The same car in the US, rated at 29.4 MPG (equivalent), with fuel at $0.95/L, costs $0.079/km. Comparing vehicles across countries requires accounting for both the fuel economy standard and the local fuel price.