Fuel Efficiency Converter

The Fundamental Problem: High Is Better for mpg, Low Is Better for L/100km

Fuel efficiency units are unusual because two of the most common ones — mpg and L/100km — are inverses of each other, and they measure efficiency in opposite directions. Miles per gallon (mpg) is a "distance per volume" metric: higher is better. A car rated at 50 mpg is more efficient than one rated at 30 mpg. Litres per 100 kilometres (L/100km) is a "volume per distance" metric: lower is better. A car rated at 5 L/100km is more efficient than one rated at 8 L/100km. This inversion means you cannot intuitively compare them — an improvement of 5 mpg at low efficiency saves far more fuel than 5 mpg at high efficiency, a phenomenon that trips up consumers evaluating car upgrade decisions. This converter shows both simultaneously so you can compare properly.

US mpg vs UK mpg: A 20% Difference With the Same Name

The most dangerous confusion in fuel economy comparison is that US and UK miles per gallon both use "mpg" but are not comparable, because the US gallon (3.785 L) and the Imperial gallon (4.546 L) differ by 20%. A European car marketed in the UK at "60 mpg" would be rated at only about 50 mpg (US) — no efficiency improvement, just a bigger gallon. This becomes critical when comparing car specs across regions, evaluating imported vehicles, or interpreting economy claims in international automotive reviews. The UK is in the process of adopting L/100km for official fuel economy figures to align with European standards, but mpg remains widely used in consumer contexts. This converter makes both mpg standards explicit, never conflating them.

Why Europe Uses L/100km and Why It Is Actually More Useful

L/100km has a practical advantage for calculating actual fuel costs: the maths is direct. If you drive 500 km and your car uses 7 L/100km, you need 500 × 7 ÷ 100 = 35 litres. Multiply by the fuel price per litre and you have the trip cost. With mpg, the same calculation requires dividing distance in miles by the mpg figure to get gallons, then multiplying by price per gallon or converting the gallon volume to litres. L/100km also scales linearly with fuel cost: cutting from 8 L/100km to 7 L/100km saves exactly 1 litre per 100 km regardless of starting point. With mpg, the fuel savings from improving mpg by a fixed amount depend heavily on where you start. For volume conversions related to fuel tank capacity, the Volume Converter handles litres, US gallons, and UK gallons.

Electric Vehicles and the End of mpg?

As electric vehicles become mainstream, fuel efficiency comparisons increasingly need to bridge petrol/diesel economy figures with EV consumption figures. The US EPA introduced "MPGe" (miles per gallon equivalent) to compare EVs with combustion vehicles — defined as the distance achievable on the energy equivalent of one US gallon of petrol (33.7 kWh). A vehicle rated at 100 MPGe uses 33.7 kWh to travel the distance a 100 mpg petrol car would cover on one gallon. Europe uses kWh/100km for EVs, analogous to L/100km. A typical EV consuming 15 kWh/100km converts to roughly 225 MPGe by US standards. Converting between these energy-based and volume-based efficiency metrics requires understanding both fuel economy units and energy units — for the energy side, the Energy Converter converts kWh to joules, BTU, and other units.

The Non-Linear Trap: Why Small mpg Gains at Low Efficiency Matter More

There is a counterintuitive mathematical truth about mpg that has policy implications: improving fuel economy from 10 to 20 mpg saves four times as much fuel as improving from 33 to 50 mpg, even though the second improvement looks larger. This is because mpg is a reciprocal function of fuel consumption. Over 10,000 miles: a 10 mpg car uses 1,000 gallons; a 20 mpg car uses 500 gallons — saving 500 gallons. A 33 mpg car uses 303 gallons; a 50 mpg car uses 200 gallons — saving only 103 gallons. L/100km captures this correctly as a linear scale. This is why environmental policy analysts use L/100km rather than mpg for evaluating the real-world impact of fleet efficiency standards. For speed conversions relevant to automotive contexts, the Speed Converter handles km/h and mph conversions.

Key Fuel Efficiency Conversion Reference

• 1 mpg (US) = 0.4251 km/L = 235.21 L/100km (reciprocal relationship)
• 1 mpg (UK) = 0.3540 km/L = 282.48 L/100km
• 1 mpg (US) = 0.8327 mpg (UK) (US gallon is smaller)
• To convert L/100km → mpg (US): divide 235.215 by L/100km value
• To convert mpg (US) → L/100km: divide 235.215 by mpg value
• 5 L/100km ≈ 47 mpg (US) ≈ 56.5 mpg (UK) ≈ 20 km/L