Fuel economy is measured in three major units worldwide: miles per gallon (mpg, US), liters per 100 kilometers (L/100km), and kilometers per liter (km/L). The converter above handles all three simultaneously — type a value in any unit and the other two update instantly. Note that mpg and km/L are efficiency ratings (higher = better), while L/100km is a consumption rate (lower = better).
The formula
L/100km is the base unit. mpg and km/L use a reciprocal relationship:
L/100km = (100 × 3.785411784) / (mpg × 1.609344)
= 235.2146 / mpg
km/L = 100 / L100km
mpg = 235.2146 / L100km
The constant 235.2146 is derived from NIST-exact values: US gallon = 3.785411784 L; 1 mile = 1.609344 km.
Practical examples
Example 1 — US to European comparison
A US car is rated at 30 mpg. A European model lists 6.5 L/100km. Which is better?
30 mpg → 235.2146 / 30 = 7.84 L/100km
7.84 vs 6.5: the European car uses less fuel. 6.5 L/100km wins.
Example 2 — Rental car fuel cost
You drive 500 km in a rental rated at 8 L/100km. Fuel costs €1.75/L.
500 km × (8/100) × 1.75 = €70 fuel cost
Example 3 — km/L to mpg for a Japanese market car
A Japanese domestic car shows 18 km/L:
L/100km = 100/18 = 5.56 → mpg = 235.2146/5.56 = 42.3 mpg
Common mistakes
Comparing mpg US with mpg Imperial. Imperial mpg is about 20% higher than US mpg for the same car because an Imperial gallon is 4.546 L vs 3.785 L. Always check which gallon is being used.
Assuming L/100km and km/L have a simple ratio. They are reciprocals of each other multiplied by 100: km/L = 100 / L100km. Treating them as directly proportional gives a wrong result.
Not accounting for ethanol blends. E10 (10% ethanol) reduces fuel economy by approximately 3–4% compared to E0 (pure petrol) because ethanol has lower energy density. Manufacturer mpg/L100km ratings typically assume the standard fuel for the market.
International and regional variations
| Region | Standard unit | Notes |
|---|---|---|
| United States | mpg (US gallons) | EPA ratings on new car stickers |
| United Kingdom | mpg (Imperial gallons) | ~20% higher numbers than US mpg |
| Europe, Canada, Australia | L/100km | Lower = more efficient |
| Japan, India | km/L | Higher = more efficient |
Quick reference — typical fuel economy by vehicle type
| Vehicle type | mpg (US) | L/100km | km/L |
|---|---|---|---|
| Large SUV / truck | 15–20 | 11.8–15.7 | 6.4–8.5 |
| Mid-size sedan | 28–35 | 6.7–8.4 | 11.9–14.9 |
| Compact car | 35–45 | 5.2–6.7 | 14.9–19.1 |
| Hybrid | 45–60 | 3.9–5.2 | 19.1–25.5 |
| Electric (equiv.) | ≥ 100 MPGe | — | — |