Fuel Economy Converter

Understanding Fuel Economy Units

Fuel economy measures how efficiently a vehicle uses fuel. Different regions use different conventions: distance-per-volume (km/L, MPG) measures how far you can go on a given amount of fuel, while volume-per-distance (L/100km) measures how much fuel is consumed over a fixed distance.

The relationship between these units is inverse: higher km/L or MPG means better economy, while lower L/100km means better economy. This inverse relationship means equal improvements in L/100km save more fuel at the inefficient end — reducing from 15 to 10 L/100km saves more fuel than reducing from 5 to 3.3 L/100km.

US gallons (3.785 L) and Imperial (UK) gallons (4.546 L) differ by about 20%, so MPG values are not directly comparable between US and UK specifications.

Official test cycle values (WLTP in Europe, EPA in the US) differ significantly in methodology. WLTP values are typically 10-20% better than EPA values for the same vehicle, so always note which test cycle a fuel economy figure comes from before converting units.

Formula: km/L = 100 / L/100km L/100km = 100 / km/L MPG (US) = km/L × 2.352 MPG (UK) = km/L × 2.825

Example Calculation

A car rated at 15 km/L: L/100km = 100/15 = 6.67. MPG (US) = 15 × 2.352 = 35.3. MPG (UK) = 15 × 2.825 = 42.4. The same fuel economy looks 20% better in UK MPG.

When to Use This Calculator

• Comparing vehicle specifications from different markets — e.g., a European L/100km rating against a US MPG rating for the same model
• Calculating fuel costs for international fleet operations where vehicles are rated in different unit systems
• Converting fuel economy data for regulatory compliance reporting that requires a specific unit (e.g., CAFE standards use MPG)
• Translating consumer-facing fuel economy labels when importing or exporting vehicles between regions

Common Mistakes to Avoid

• Using US MPG values as if they were UK MPG — Imperial gallons are 20% larger, so UK MPG numbers are always higher for the same vehicle; mixing them up gives a 20% error
• Linearly averaging MPG values — the average of 20 and 40 MPG is not 30 MPG in terms of fuel consumed; use the harmonic mean or convert to L/100km first, then average
• Comparing WLTP and EPA values directly — WLTP test cycle results are typically 10-20% more optimistic than EPA; always note the test standard before comparing
• Forgetting that fuel economy varies with driving conditions — a highway rating of 5 L/100km can double to 10 L/100km in stop-and-go city driving; conversions only apply to the same driving condition

How to Interpret Results

• km/L and MPG are distance-per-volume scales where higher = better; L/100km is volume-per-distance where lower = better — the two scales are inverses of each other
• UK MPG always reads ~20% higher than US MPG for the same vehicle because Imperial gallons (4.546 L) are larger than US gallons (3.785 L); never compare UK and US MPG figures directly
• For fleet cost analysis, convert all figures to L/100km first — this unit is linear and can be averaged or summed across vehicles; MPG values cannot be averaged directly
• Official WLTP and EPA test values are not directly comparable even after unit conversion — WLTP cycle conditions differ from EPA; always note the test standard when converting

Related Standards & References

• WLTP (Worldwide Harmonised Light Vehicle Test Procedure) — Global standard for fuel consumption and CO₂ measurement adopted by the EU from 2017, replacing NEDC
• US EPA Fuel Economy Test — FTP-75 (city) combined with HWFET (highway) cycles; forms the basis for US fuel economy window sticker ratings
• SAE J1711 — Recommended Practice for Measuring the Exhaust Emissions and Fuel Economy of Hybrid-Electric Vehicles, Including Plug-in Hybrid Vehicles
• EU Regulation 2017/1151 — Implementing WLTP for CO₂ emissions and fuel consumption type approval in the European Union

Frequently Asked Questions

Why do European cars use L/100km instead of km/L?

L/100km is linear with fuel consumption — double the fuel use means double the number. This makes it easier to compare costs and CO₂ emissions directly. km/L and MPG are hyperbolic, making differences at the inefficient end appear smaller than they actually are.

How do hybrid and EV efficiency compare to gasoline?

Hybrids achieve 20-25 km/L (47-59 MPG US). EVs use kWh/100km instead, typically 15-20 kWh/100km. Converting to equivalent fuel economy: 1 kWh ≈ 0.09 L gasoline equivalent, so 15 kWh/100km ≈ 1.35 L/100km equivalent (74 km/L).

Why does my real-world fuel economy differ from the official rating?

Official test cycles are conducted under controlled conditions with moderate speeds, no climate control, and a warm engine. Real-world driving includes cold starts, air conditioning, highway speeds above the test cycle, traffic congestion, and cargo weight — all of which reduce fuel economy by 10-30% compared to WLTP or EPA ratings.