Fuel efficiency is reported in incompatible units across countries and industries — MPG in the US (using US gallons), MPG-Imperial in the UK (using Imperial gallons), L/100km across most of Europe, and km/L in Japan and much of Asia. These differences mean a "45 MPG" claim requires knowing which system the number references, or you can be off by 20% comparing vehicles across markets. Electric vehicles add a fifth unit, MPGe, that estimates how many miles a liquid-fuel vehicle would travel on the energy equivalent of one gallon of gasoline. The sections below cover the non-linear MPG-to-L/100km relationship and why policy experts prefer L/100km, the EU efficiency tier system this calculator applies, the gap between rated and real-world efficiency, and how to interpret MPGe for electric vehicles.
MPG vs. L/100km: A Non-Linear Relationship
MPG and L/100km are inversely related — higher MPG means lower fuel consumption per distance — and this inverse relationship produces a non-linear effect that surprises many consumers. Improving from 15 to 20 MPG saves far more fuel per mile than improving from 35 to 40 MPG, even though both look like "5 MPG improvements" on the spec sheet. Specifically, going from 15 to 20 MPG saves about 1.67 gallons per 100 miles; going from 35 to 40 MPG saves only 0.36 gallons per 100 miles — a 4.6× larger fuel savings from the same nominal MPG change.
This non-linearity is why many policy experts, environmental economists, and European regulators prefer L/100km — it scales linearly with actual fuel consumption and makes fuel savings directly comparable across vehicles. Going from 15 L/100km to 12 L/100km saves exactly 3 L per 100 km, and this 3-L savings is directly comparable regardless of whether the vehicle started at 20 L/100km or 10 L/100km. The US EPA publishes "gallons per 100 miles" alongside MPG on new-car stickers for similar reasons, and this linear-consumption metric is more useful for estimating annual fuel costs than MPG alone. When comparing replacement vehicles, always calculate annual fuel cost at your typical mileage rather than relying on MPG as a standalone efficiency indicator.
The EU Efficiency Label Explained
The A+ to F rating shown by this converter is adapted from EU appliance energy labelling standards and applied to vehicles by MPG US threshold, providing a consistent visual tier across the full efficiency spectrum. A+ (≥55 MPG US) covers the best hybrids and electric vehicles — Toyota Prius Prime plug-in models, Tesla Model 3 (as measured by MPGe), and the most efficient compact hybrids land in this tier. A tier (45–54 MPG) includes most standard hybrid sedans and some efficient diesel vehicles.
B tier (35–44 MPG) covers well-designed gasoline compact cars and small hybrids. C tier (25–34 MPG) represents the average new car sold in the US — the typical midsize sedan, compact SUV, or entry-level crossover falls here. D tier (17–24 MPG) contains most midsize SUVs, pickup trucks, and larger sedans. E tier (12–16 MPG) covers full-size SUVs, larger pickups, and sports cars. F tier (<12 MPG) includes heavy-duty trucks, large commercial vehicles, and exotic performance cars that prioritize acceleration over efficiency. This tier system makes vehicle comparison visceral — shopping for a "C tier" vs "A tier" vehicle conveys the efficiency tradeoff more immediately than comparing raw MPG numbers across dramatically different vehicle classes.
Real-World vs. Rated Efficiency
EPA laboratory test-cycle figures typically differ from on-road performance by 10–25% because standardized test cycles don't capture the full range of real-world driving conditions. Aggressive driving (rapid acceleration, hard braking, high speeds above 65 mph) reduces efficiency 15–25%, cold weather cuts 10–20% (partly because of battery performance for hybrids, partly because of warm-up fuel consumption), roof racks and cargo carriers add aerodynamic drag costing 5–25%, and low tire pressure (2–3 psi below spec) can reduce efficiency 5–10%.
When comparing vehicles across international specs, use the same test standard. The US EPA uses a city/highway/combined cycle. Europe uses WLTP (Worldwide Harmonised Light Vehicle Test Procedure, replaced NEDC in 2017). Japan uses JC08, transitioning to WLTP. Comparing an "EU cycle 6.5 L/100km" vehicle to a "US EPA 40 MPG" vehicle requires either converting both to a common standard or acknowledging the test methodologies aren't directly comparable. For personal planning, track your actual on-road efficiency over 3–6 months using fuel receipts and odometer readings — your individual driving pattern usually produces 5–15% worse MPG than EPA combined ratings for most vehicles.
Electric Vehicles and MPGe
MPGe (miles per gallon equivalent) allows electric vehicles to be compared on the same energy-efficiency scale as combustion vehicles by expressing EV consumption in gasoline-equivalent units. The EPA defines 33.7 kWh of electrical energy as equivalent to one gallon of gasoline — this conversion factor comes from the lower heating value of gasoline (around 115,000 BTU per gallon ÷ 3,412 BTU per kWh = 33.7 kWh). A Tesla Model 3 Long Range rated at 141 MPGe consumes about 24 kWh per 100 miles, dramatically less than any gasoline vehicle because EV powertrains are 85%+ efficient at converting stored energy to motion while combustion engines typically waste 60–75% of fuel energy as heat.
MPGe has limitations as a comparison metric, though, because it doesn't account for the energy source of the electricity (coal-heavy grids produce dramatically more CO₂ per mile than renewable-heavy grids, even at identical MPGe) or the full manufacturing energy footprint (EV battery manufacturing is energy-intensive upfront, offsetting some operational efficiency gains). For honest total-impact comparisons, use lifecycle analysis tools that integrate grid mix, manufacturing energy, and vehicle lifespan. For immediate operational cost comparisons, MPGe combined with electricity rates gives a clean dollars-per-mile number that's usually 3–5× lower than gasoline vehicles in most US markets.