Fuel Economy Converter

What Is a Fuel Economy Converter?

Convert between miles per gallon (MPG), liters per 100km, and kilometers per liter. This calculator runs entirely in your browser — your data stays private, and no account is required.

Understanding Fuel Economy Units

Fuel economy is expressed differently worldwide: the U.S. uses miles per gallon (MPG), most of the world uses liters per 100 kilometers (L/100km), and some countries use kilometers per liter. These systems are inversely related — higher MPG means better efficiency, while lower L/100km means better efficiency.^[1] U.S. and UK gallons are different sizes (1 US gallon = 3.785 L vs 1 Imperial gallon = 4.546 L), so a car rated at 40 MPG in the U.S. would be rated at approximately 48 MPG Imperial in the UK.^[2] The EPA tests for fuel economy using standardized city and highway driving cycles, but real-world MPG is typically 10-20% lower than the EPA estimate due to driving habits, weather, and road conditions.^[3] Use the Gas Cost Calculator to estimate your fuel expenses.

Real-World vs EPA Estimates

EPA fuel economy estimates are lab-based and typically 15–25% above real-world driving. Highway driving is more fuel-efficient than city driving due to less acceleration and braking. Driving at 55 mph versus 70 mph improves fuel economy by 10–15%. Properly inflated tires add 0.5–3% efficiency. Air conditioning increases fuel use by 5–25% depending on conditions. EVs are more efficient in city driving because regenerative braking recovers energy that gasoline cars waste as heat.

Fuel Economy Unit Conversions

Understanding Fuel Economy Measurements

Fuel economy is measured differently around the world, creating confusion when comparing vehicles across markets. The United States uses miles per gallon (MPG), most other countries use liters per 100 kilometers (L/100km), and the United Kingdom uses miles per imperial gallon (which is larger than a U.S. gallon). Converting between these systems requires understanding both the distance and volume unit differences.

Key Conversion Formulas

MPG (US) to L/100km: Divide 235.214 by the MPG value. A car getting 30 MPG = 235.214 ÷ 30 = 7.84 L/100km. L/100km to MPG (US): Divide 235.214 by the L/100km value. A car rated at 6 L/100km = 235.214 ÷ 6 = 39.2 MPG. MPG (US) to MPG (UK/Imperial): Multiply by 1.201 (because a UK gallon is 20% larger than a US gallon). A car getting 30 US MPG = 36 UK MPG. The inverse relationship in MPG vs. L/100km is important to understand: in MPG, a higher number is better (more miles per gallon). In L/100km, a lower number is better (fewer liters consumed per 100 km). This reversal often confuses people switching between systems.

Why L/100km Is More Useful Than MPG

The L/100km metric is mathematically superior for comparing fuel costs because it scales linearly with fuel consumption. In MPG, the relationship is hyperbolic — improving from 10 to 20 MPG saves far more fuel than improving from 30 to 40 MPG. Specifically: driving 10,000 miles at 10 MPG burns 1,000 gallons; at 20 MPG, 500 gallons (saving 500 gallons). But at 30 MPG you burn 333 gallons, and at 40 MPG you burn 250 gallons (saving only 83 gallons). This MPG illusion causes consumers to undervalue improvements in low-MPG vehicles (trucks, SUVs) and overvalue improvements in already-efficient vehicles. A fleet manager replacing 20-MPG trucks with 25-MPG trucks saves more fuel than replacing 35-MPG sedans with 50-MPG hybrids — a fact obscured by the MPG metric but immediately obvious in L/100km.

Real-World vs. EPA Ratings

EPA fuel economy ratings are determined through laboratory testing on a dynamometer, not real-world driving. While the EPA adjusted its testing protocol in 2008 to better reflect real driving conditions, actual fuel economy still typically falls 10–20% below the sticker MPG for most drivers. Factors that reduce real-world economy include aggressive acceleration (reduces economy by 15–30%), highway speeds above 60 mph (every 5 mph above 50 costs roughly $0.20–$0.30 per gallon in efficiency loss), cold weather (reduces economy by 15–25% in gasoline vehicles, up to 40% in EVs), use of roof racks and cargo boxes (10–25% reduction at highway speeds due to aerodynamic drag), and underinflated tires (0.2% economy loss per 1 PSI below recommended pressure).

Electric Vehicle Efficiency

EVs use miles per gallon equivalent (MPGe) in the U.S. — where 33.7 kWh of electricity is defined as equivalent to one gallon of gasoline based on energy content. A Tesla Model 3 rated at 132 MPGe means it travels 132 miles on 33.7 kWh, or about 0.255 kWh per mile (255 Wh/mi). The international metric for EVs is kWh per 100 km — directly analogous to L/100km. A car consuming 15 kWh/100km is more efficient than one consuming 20 kWh/100km. To compare fuel costs between gas and electric: if electricity costs $0.15/kWh and gasoline costs $3.50/gallon, an EV consuming 0.3 kWh/mile costs $0.045/mile while a 30 MPG gasoline car costs $0.117/mile — making the EV about 62% cheaper per mile in energy costs.

Hypermiling and Efficiency Tips

Practical techniques to improve fuel economy: maintain steady speed using cruise control on highways (5–7% improvement). Anticipate traffic flow to minimize braking and acceleration. Remove unnecessary weight — every 100 lbs reduces economy by approximately 1%. Keep tires inflated to the recommended PSI (found on the driver's door jamb, not on the tire sidewall). Use the recommended motor oil grade — switching from 10W-40 to the manufacturer-specified 5W-20 can improve economy by 1–2%. Combine short trips (a cold engine uses significantly more fuel) and avoid idling longer than 30 seconds. These techniques collectively can improve real-world fuel economy by 15–25% compared to typical driving habits.

Calculating Annual Fuel Costs

To estimate annual fuel spending: divide your expected annual mileage by the vehicle's real-world MPG, then multiply by the per-gallon fuel price. The average American drives approximately 13,500 miles per year. At 25 MPG and $3.50/gallon, that equals 540 gallons × $3.50 = $1,890 per year. At 35 MPG, the same mileage costs $1,350 — saving $540 annually. Over a typical 5-year ownership period, that fuel savings ($2,700) often exceeds the price difference between a standard and hybrid trim level. For a more precise estimate, factor in your actual driving split between city and highway — EPA city and highway ratings can differ by 5–10 MPG, and the combined rating uses a weighted average of 55% city and 45% highway driving. If your commute is primarily highway, your real-world economy will likely exceed the combined rating.

This converter handles all major fuel economy unit conversions including MPG, L/100km, km/L, and MPGe, making international vehicle comparisons straightforward and accurate regardless of which measurement system you start from.

See also: Gas Mileage Calculator · Gas Cost Calculator · EV vs Gas Car Calculator · Speed Converter