MPG vs L/100km: Understanding Fuel Economy Numbers

If you've ever compared cars from different countries, you've probably noticed that fuel economy is measured differently. The US uses miles per gallon (MPG), while most of the world uses liters per 100 kilometers (L/100km). Understanding both is crucial for making informed vehicle comparisons.

The Key Difference: Higher vs Lower is Better

This is the most confusing aspect: with MPG, a higher number means better fuel economy. With L/100km, a lower number means better fuel economy. They're inversely related: - 30 MPG (US) ≈ 7.84 L/100km - 40 MPG (US) ≈ 5.88 L/100km - 50 MPG (US) ≈ 4.70 L/100km

The Conversion Formula

To convert between these units: MPG (US) = 235.215 ÷ L/100km, and L/100km = 235.215 ÷ MPG (US). Note that US gallons and Imperial gallons are different sizes — use 282.481 for Imperial MPG conversions.

Why L/100km is More Intuitive

Mathematicians and engineers often argue that L/100km is more practical because it directly tells you how much fuel you'll use for a trip. If you're driving 500 km and your car uses 6 L/100km, you need 30 liters. With MPG, the math is less straightforward.

The MPG Illusion

Going from 10 MPG to 20 MPG saves more fuel than going from 30 MPG to 60 MPG for the same distance driven. This "MPG illusion" happens because MPG is inversely proportional to fuel consumption. A car getting 10 MPG uses 10 gallons per 100 miles, while 20 MPG uses 5 gallons — saving 5 gallons. But 30 MPG uses 3.33 gallons, and 60 MPG uses 1.67 gallons — only saving 1.67 gallons.

Typical Fuel Economy by Vehicle Type

Reference values (approximate US MPG / L/100km): - Small car: 35 MPG / 6.7 L/100km - Mid-size sedan: 30 MPG / 7.8 L/100km - SUV: 25 MPG / 9.4 L/100km - Pickup truck: 20 MPG / 11.8 L/100km - Hybrid: 50 MPG / 4.7 L/100km - Electric (equivalent): 100+ MPGe / <2 L/100km equivalent