Fuel Economy Calculator
Fuel economy conversion between miles per gallon (US/UK) and litres per 100 kilometres
Last updated:
What this calculator computes
Fuel-economy conversion is uniquely tricky because the US-customary "miles per gallon" and the metric "litres per 100 kilometres" are inverse measures of each other — distance per unit fuel versus fuel per unit distance — and the inverse relationship means doubling one value does not halve the other. The calculator handles three primary conversions: US miles per gallon (mpg) to litres per 100 km (L/100 km), UK imperial miles per gallon to L/100 km (with the imperial-vs-US gallon distinction adding a 20% scaling factor), and the inverse direction from L/100 km back to either mpg variant. The conversion factor for US mpg to L/100 km is 235.215, derived from the US gallon (3.785 L) and the mile-to-km factor (1.609344) combined into a single multiplier that handles the inverse-relationship math automatically. The L/100 km figure is more linear in real-world fuel-cost terms because doubling the figure doubles the fuel consumed for a given distance, while doubling the mpg figure halves it; this is why most government fuel-efficiency standards (EU, Japan, Korea) report in L/100 km even though consumer marketing often retains mpg-equivalent figures. EV ratings introduce a fourth metric — kWh per 100 km or miles per kWh equivalent — that the calculator handles via the EPA-published electrical-energy-to-fuel-energy conversion (1 gallon of gasoline ≈ 33.7 kWh of energy).
Calculator
The formula
Formula
L/100 km = 235.215 / mpg(US) mpg(US) = 235.215 / L/100 km
Worked example
When to use this calculator
Use this calculator when comparing US-rated vehicles against European or Asian-rated competitors, when reading European fuel-economy specifications for vehicles also sold in the US market, or when interpreting global vehicle reviews that mix unit systems within a single article. The calculator is essential for cross-shopping decisions where the underlying physics (fuel consumed per distance travelled) is the comparison target but the published figures use different unit conventions. EV ratings introduce additional complications: US EV ratings show "MPGe" (mpg-equivalent based on energy content) and miles per kWh, while European ratings show kWh per 100 km — the calculator handles the conversion via the EPA-published 33.7 kWh-per-gallon energy equivalent. The calculator does not handle real-world-vs-rated economy, where individual driving styles and conditions can produce 20–30% deviation from the rated figures regardless of which unit system the rating uses.
Common input mistakes
- Using a linear conversion (multiply or divide by a constant) instead of the inverse-relationship formula. mpg and L/100 km are inverse measures, so a 30 mpg figure does not become 7.84 L/100 km via simple multiplication — the correct conversion divides 235.215 by the mpg figure. Linear-scaling errors produce wildly wrong answers, especially at the high and low ends of the efficiency range.
- Confusing US-gallon mpg with UK imperial-gallon mpg. The two gallons differ by about 20%, with the UK imperial gallon being larger, so a "40 mpg" UK figure equals about 33 mpg in US units. Cross-publication reviews that fail to specify which gallon is meant introduce a 20% efficiency error, large enough to fundamentally change the cross-vendor comparison.
Frequently asked questions
How do I convert mpg to L/100 km?
Divide 235.215 by the US mpg figure to get L/100 km. A 30 mpg figure becomes 7.84 L/100 km; a 40 mpg figure becomes 5.88 L/100 km; a 50 mpg figure becomes 4.70 L/100 km. The 235.215 constant combines the US gallon-to-litre factor (3.785) with the mile-to-km factor (1.609344) and the per-100-km scaling, producing a single multiplier that handles the inverse-relationship math.
Why is L/100 km more useful than mpg for cost comparison?
L/100 km is linear in fuel consumed for a given distance, so doubling the figure doubles the fuel cost per trip. mpg is the inverse, so improving from 30 mpg to 40 mpg (a 33% mpg increase) saves the same fuel as improving from 50 mpg to 100 mpg (a 100% mpg increase) — both reduce fuel use by the same absolute amount per mile. Government fuel-efficiency ratings worldwide outside the US use L/100 km or its equivalent because the linearity makes cost-savings comparisons more intuitive.
How does UK mpg differ from US mpg?
UK miles per gallon uses the imperial gallon (4.546 L), about 20% larger than the US gallon (3.785 L), so the same physical fuel-economy figure produces UK-mpg numbers about 20% higher than US-mpg numbers. A vehicle rated 40 mpg in UK specifications is approximately 33 mpg in US specifications. Cross-shopping British and US car reviews requires confirming which gallon the figure references, because the 20% gap is large enough to fundamentally change the cross-comparison conclusion.
How are EV efficiency ratings converted?
EV efficiency in the US is reported as MPGe (mpg-equivalent based on the EPA's 33.7 kWh-per-gallon energy-content conversion) and miles per kWh. EU EV ratings use kWh per 100 km. To convert miles per kWh to kWh per 100 km, divide 62.137 (the km-per-mile-times-100 factor) by the miles-per-kWh figure. A 4 miles-per-kWh EV equals 15.5 kWh per 100 km, equivalent to about 105 MPGe in US units.
How accurate are EPA and EU fuel-economy ratings?
Real-world fuel economy typically deviates 10–20% from rated figures due to driving style, traffic conditions, climate, terrain, and accessory load. EPA ratings are derived from a standardised test cycle that includes city, highway, and combined driving; EU ratings use the WLTP cycle, which has been calibrated to be more representative of real-world driving than the older NEDC cycle. Both rating systems remain useful for cross-vehicle comparison even though absolute consumption may differ from individual driving outcomes.