MPG vs L/100km: Why Fuel Economy Units Confuse Everyone

The Fuel Economy💡 Definition:Frugality is the practice of mindful spending to save money and achieve financial goals. Confusion

If you have ever tried to compare car specifications across different countries, you have encountered the frustrating reality that fuel economy is measured in completely different ways:

• United States: Miles per gallon💡 Definition:Distance traveled per unit of fuel consumed (MPG)
• United Kingdom: Miles per gallon (but larger gallons!)
• Europe, Australia: Liters per 100 kilometers (L/100km)
• Japan, India: Kilometers per liter (km/L)

These are not just different units—they represent fundamentally different ways of thinking about fuel efficiency.

MPG: Distance Per Fuel

Miles per gallon tells you how far you can travel on a fixed amount of fuel. Higher numbers are better. This is intuitive for asking "how far can I go on a tank?"

L/100km: Fuel Per Distance

Liters per 100 kilometers tells you how much fuel you need for a fixed distance. Lower numbers are better. This is intuitive for asking "how much will💡 Definition:A will is a legal document that specifies how your assets should be distributed after your death, ensuring your wishes are honored. this trip cost?"

Why L/100km Is Mathematically Better

Here is the crucial insight most people miss: MPG is deceptive when comparing efficiency improvements.

Consider these scenarios:

Scenario 1: You upgrade from a 15 MPG truck to a 20 MPG SUV Scenario 2: You upgrade from a 35 MPG sedan to a 50 MPG hybrid

Which saves more fuel over 10,000 miles?

Most people guess Scenario 2 (bigger MPG jump). They are wrong.

The Math

Scenario 1 (15 → 20 MPG):

• Old: 10,000 ÷ 15 = 667 gallons
• New: 10,000 ÷ 20 = 500 gallons
• Savings: 167 gallons

Scenario 2 (35 → 50 MPG):

• Old: 10,000 ÷ 35 = 286 gallons
• New: 10,000 ÷ 50 = 200 gallons
• Savings: 86 gallons

The smaller MPG improvement saves nearly twice as much fuel! This happens because MPG has a hyperbolic relationship with fuel consumption.

In L/100km, these would be:

• 15 MPG = 15.7 L/100km → 20 MPG = 11.8 L/100km (3.9 L/100km improvement)
• 35 MPG = 6.7 L/100km → 50 MPG = 4.7 L/100km (2.0 L/100km improvement)

Now the actual fuel savings are obvious!

US Gallons vs Imperial Gallons

Making things more confusing, the US and UK use different sized gallons:

• US gallon: 3.785 liters
• Imperial gallon: 4.546 liters (20% larger)

This means:

• 30 MPG (US) = 36 MPG (UK)
• A car rated 40 MPG in the UK would be rated only 33 MPG in the US

Always check which gallon is being referenced when comparing specifications.

Quick Conversion Formulas

MPG to L/100km

L/100km = 235.215 ÷ MPG (US) L/100km = 282.481 ÷ MPG (UK)

L/100km to MPG

MPG (US) = 235.215 ÷ L/100km MPG (UK) = 282.481 ÷ L/100km

US to UK MPG

MPG (UK) = MPG (US) × 1.201

Common Fuel Economy Values

*Electric vehicles use MPGe (equivalent) based on energy content of gasoline

Why Real-World Economy Differs from EPA Ratings

EPA ratings are tested under controlled conditions. Real-world factors that reduce fuel economy:

• Driving style: Aggressive acceleration/braking can reduce economy by 15-30%
• Speed: Fuel economy drops significantly above 50 mph
• Temperature: Cold engines and HVAC use reduce economy
• Terrain: Hills require more energy
• Cargo: Extra weight reduces efficiency
• Tire pressure: Low pressure increases rolling resistance
• Traffic: Stop-and-go driving is less efficient than highway

Tips for Better Fuel Economy

1. Maintain steady speeds using cruise control
2. Accelerate and brake gently
3. Keep tires properly inflated
4. Remove unnecessary weight
5. Reduce air conditioning use when possible
6. Combine short trips
7. Keep up with maintenance

Use our Fuel Economy Converter to compare specifications and calculate your actual fuel costs.