How It Works

Fuel efficiency tells you how much fuel your vehicle uses to travel a certain distance. Different countries use different units, which makes comparing cars or planning trips across borders confusing.

This calculator converts between four common units instantly:

• L/100km (liters per 100 kilometers): Used in most of Europe, Australia, and Canada. Lower numbers mean better efficiency.
• MPG US (miles per gallon, US): Used in the United States. Higher numbers mean better efficiency.
• MPG UK (miles per gallon, Imperial): Used in the United Kingdom. Higher numbers mean better efficiency. UK gallons are about 20% larger than US gallons.
• km/L (kilometers per liter): Used in some Asian and Latin American countries. Higher numbers mean better efficiency.

The math behind conversions uses these relationships:

MPG (US) = 235.214 ÷ L/100km
MPG (UK) = 282.481 ÷ L/100km
km/L = 100 ÷ L/100km

Why MPG and L/100km Feel Backwards

People often get confused because MPG and L/100km move in opposite directions. When MPG goes up, your car is using less fuel (good). When L/100km goes up, your car is using more fuel (bad).

Think of it this way: MPG tells you “how far can I go with this fuel?” L/100km tells you “how much fuel do I need for this distance?” They’re answering different questions about the same thing.

What If My Car Shows Different Numbers?

Car computers aren’t perfect. They estimate fuel consumption based on engine sensors, not actual fuel used. Real-world efficiency depends on driving style, terrain, weather, tire pressure, and vehicle load.

Your dashboard might show 7.5 L/100km, but you could actually be using 8.2 L/100km in city traffic or 6.8 L/100km on the highway. To get your true consumption, divide liters filled by kilometers driven since the last fill-up, then multiply by 100.

Converting Between US and UK Gallons

A US gallon is 3.785 liters. A UK gallon is 4.546 liters. That 20% difference matters when comparing fuel economy specs.

A car rated at 30 MPG (US) is actually doing 36 MPG (UK). Same car, same efficiency, just different gallon sizes. Always check which MPG standard a manufacturer is using, especially when comparing European and American vehicles.

Common Values at a Glance

Calculating Fuel Costs From Efficiency

Once you know your fuel consumption, calculating trip costs is straightforward. Multiply your consumption rate by the distance you’re traveling, then multiply by the fuel price.

Example: You’re driving 450 km in a car that uses 8.0 L/100km. Fuel costs $1.50 per liter. Your calculation: (450 ÷ 100) × 8.0 × 1.50 = $54.00 for the trip.

For MPG calculations in the US, divide miles by MPG to get gallons needed, then multiply by price per gallon. A 280-mile trip at 28 MPG with $3.50/gallon gas: (280 ÷ 28) × 3.50 = $35.00.