Fuel Economy Converter

Fuel economy measures how efficiently a vehicle converts fuel into distance. Two fundamentally different approaches exist: the United States uses miles per gallon (MPG) — a higher number means better efficiency. Most of the world uses liters per 100 kilometers (L/100km) — a lower number means better efficiency. These are inverse (reciprocal) measures, so comparing them requires a formula, not simple multiplication.

The relationship between MPG and L/100km is not linear, which creates a common misconception. Improving from 20 MPG to 25 MPG saves more fuel over the same distance than improving from 40 MPG to 50 MPG. In L/100km terms this is clear: 20 MPG ≈ 11.76 L/100km; 25 MPG ≈ 9.41 L/100km (saving 2.35 L per 100km). Meanwhile, 40 MPG ≈ 5.88 and 50 MPG ≈ 4.70 (saving only 1.18 L per 100km). The L/100km system makes these tradeoffs visible; the MPG system obscures them.

UK Imperial MPG uses the Imperial gallon (4.546 liters), which is 20% larger than the US gallon (3.785 liters). A car rated at 40 UK MPG achieves only about 33 US MPG — the same distance, but measured against a larger gallon. When reading reviews of cars sold in both markets, always check which gallon standard applies. European reviews of UK cars often show both UK MPG and L/100km; American reviews show only US MPG.

Electric vehicles are rated in different units: the US uses MPGe (miles per gallon equivalent) or kWh/100mi, while most other countries use kWh/100km. The EPA defines 1 MPGe as the efficiency equivalent of a car achieving 1 US MPG on gasoline, where 33.7 kWh equals the energy content of 1 US gallon. A Tesla Model 3 rated at 134 MPGe uses approximately 25 kWh per 100 miles, or about 15.5 kWh per 100 km.