Fuel Economy Converter

How to Use the Fuel Economy Converter

Enter a fuel economy value, choose its unit and the unit to convert to, and the result updates instantly. Note that L/100km measures fuel used per distance (lower is better), the opposite direction of mpg and km/L (higher is better), so this tool automatically handles that inverse relationship for you rather than requiring a manual reciprocal calculation.

Worked Examples

• 30 mpg (US) to L/100km: 30 mpg (US) = 7.840486 L/100km, a typical figure for a mid-size gasoline sedan.
• 30 mpg (US) to km/L: 30 mpg (US) = 12.754311 km/L.
• 30 mpg (US) to mpg (UK): 30 mpg (US) = 36.028498 mpg (UK), since the UK's larger imperial gallon makes the same fuel efficiency look like a bigger number.
• 8 L/100km to km/L: 8 L/100km = 12.5 km/L, since both figures describe the exact same fuel efficiency from opposite directions.
• 15 km/L (a fuel-efficient hatchback) to mpg (US): 15 km/L = 35.282188 mpg (US).

Common Fuel Economy Units Explained

MPG (US) uses the US gallon (3.785411784 liters) and is the standard fuel economy unit on American new-vehicle window stickers and official EPA ratings.

MPG (UK) uses the larger imperial gallon (4.54609 liters), still commonly quoted in the UK and a few Commonwealth countries even though most of Europe has moved to L/100km.

km/L (kilometers per liter) is the metric equivalent of mpg, distance traveled per unit of fuel, common in India, Japan, and several other markets.

L/100km (liters per 100 kilometers) is the standard fuel economy unit across most of Europe and inverts the usual logic: it measures fuel consumed over a fixed distance, so a lower number means better efficiency, the opposite of every other unit on this page.

Common Mistakes When Converting Fuel Economy

The most common mistake is treating L/100km like the other three units, assuming a bigger number means better efficiency. It's inverted: 5 L/100km is more efficient than 8 L/100km, while for mpg and km/L a bigger number is always better. This tool's internal math (100 divided by the value, rather than a simple multiplication) handles that inversion automatically, so you don't need to remember which direction is "better" when reading the result.

Another mistake is comparing a US mpg figure directly against a UK mpg figure without converting, since the two use different-sized gallons. A car rated "40 mpg" in a UK review is only about 33.3 mpg by US standards, a meaningful difference when cross-shopping vehicles between US and UK sources.

Also be careful comparing official lab-tested fuel economy ratings against real-world trip computer readings. Lab tests use controlled, idealized conditions, so actual on-road fuel economy is typically 10 to 20% worse than the official rating, more in heavy traffic, cold weather, or with a heavily loaded vehicle.

Real-World Use Cases

Car shopping across markets: comparing a US-market car's mpg rating against a European-market car's L/100km rating, or a UK review's mpg (UK) figure against a US mpg (US) spec.

Trip planning: estimating fuel needed and cost for a road trip by converting a car's rated efficiency to L/100km and multiplying by the planned distance, then multiplying the result by your local fuel price to budget the trip.

Fleet and logistics: standardizing fuel efficiency figures across a mixed fleet of vehicles sourced from different countries with different rating conventions, so cost-per-km comparisons stay apples-to-apples.

Automotive research and reviews: converting manufacturer specs into whichever unit is standard for your audience or publication, whether you're writing for a US audience that expects mpg or an international audience that expects L/100km.

Frequently Asked Questions

Why is L/100km the opposite of mpg?

Miles per gallon and km/L measure distance per unit of fuel (higher is more efficient), while L/100km measures fuel used per fixed distance (lower is more efficient). They're inversely related, and this tool converts between them automatically.

Why are US mpg and UK mpg different?

A US gallon (3.785 liters) is smaller than a UK imperial gallon (4.546 liters), so the same physical fuel efficiency produces a higher mpg figure in the UK than in the US. Always check which gallon a source is using before comparing mpg figures.

How do I convert a car's official mpg rating to how much fuel it'll use on a specific trip?

Convert the mpg rating to L/100km using this tool, then multiply by (trip distance in km / 100) to get liters needed. For example, a car rated at 30 mpg (US) converts to about 7.84 L/100km, so a 250 km trip would use roughly 19.6 liters of fuel, before accounting for real-world factors like traffic, terrain, and driving style that typically push actual consumption above the official rating.

Why do official fuel economy ratings rarely match real-world mileage?

Official mpg and L/100km ratings come from standardized lab tests (like the US EPA cycle or Europe's WLTP cycle) run under controlled conditions: flat terrain, steady speeds, moderate temperature, no air conditioning load. Real-world driving involves hills, traffic, weather, cargo weight, and aggressive acceleration, all of which reduce actual fuel economy, typically by 10 to 20% compared to the official rating, sometimes more in city stop-and-go traffic.

Can this converter handle electric vehicle efficiency figures like miles/kWh?

No, this tool only converts between liquid-fuel efficiency units (mpg US, mpg UK, km/L, L/100km). Electric vehicle efficiency is measured in fundamentally different units like miles per kWh or kWh per 100 km, since electricity is measured in energy (kWh) rather than volume (gallons or liters), and isn't included in this converter.

What's the difference between city, highway, and combined mpg ratings?

City mpg reflects stop-and-go driving with frequent braking and idling, which lowers fuel economy. Highway mpg reflects steady-speed cruising, which is typically more efficient. Combined mpg is a weighted average of the two (roughly 55% city, 45% highway in the US EPA methodology) meant to represent typical mixed driving. This converter works with whichever single figure you enter, so convert the specific city, highway, or combined number you're interested in rather than mixing them.