MPG vs. L/100 km: Converting Fuel Economy and Why MPG Misleads
October 21, 2026
US miles per gallon and metric litres per 100 km measure the same thing in opposite ways. MPG is distance per unit of fuel, so higher is better, while L/100 km is fuel per unit of distance, so lower is better. To convert, divide 235.215 by the value: 30 mpg equals 235.215 ÷ 30 ≈ 7.84 L/100 km, and the formula works in both directions.
Skip the math and get your answer instantly:
Open the Speed Calculator →Conversion Table (US Gallons)
The constant 235.215 comes from 3.78541 litres per US gallon and 1.609344 kilometres per mile: 100 × 3.78541 ÷ 1.609344 = 235.215. UK imperial gallons are bigger (4.54609 litres), so the constant for imperial mpg is 282.481.
| US mpg | L/100 km | Gallons per 100 miles |
|---|---|---|
| 20 | 11.76 | 5.00 |
| 25 | 9.41 | 4.00 |
| 30 | 7.84 | 3.33 |
| 40 | 5.88 | 2.50 |
| 50 | 4.70 | 2.00 |
The MPG Illusion
Because MPG is distance per fuel, equal MPG gains do not mean equal fuel savings. Over 100 miles, a car that goes from 10 mpg to 20 mpg drops from 10 gallons to 5, saving 5 gallons. A car going from 30 mpg to 40 mpg drops from 3.33 gallons to 2.5, saving only 0.83 gallons.
This is why replacing a very inefficient vehicle saves far more fuel than upgrading an already efficient one, and why fuel-per-distance is a fairer way to compare. Fuel use is what you actually pay for.
Comparing Cars Fairly
To estimate annual fuel cost, divide yearly distance by mpg and multiply by the price per gallon. For 12,000 miles at 30 mpg, you need 400 gallons; at $3.50 per gallon that is $1,400. At 40 mpg you need 300 gallons and spend $1,050, saving $350.
For electric vehicles, efficiency is shown as kWh per 100 miles or miles per kWh, and MPGe converts using 33.7 kWh as the energy in a gallon of gasoline.
Frequently Asked Questions
How do I convert mpg to L/100 km?
Divide 235.215 by the mpg figure (for US gallons). For imperial mpg, divide 282.481 by the figure.
Is 30 mpg good?
It is around average for a modern gasoline car. It equals about 7.8 L/100 km.
Why is a lower L/100 km better?
It measures fuel used per distance, so lower means less fuel for the same trip.
Are UK and US mpg the same?
No. A UK gallon is about 20% larger, so a UK mpg figure is about 1.2 times its US equivalent.