PRACTICAL GUIDE / REVIEWED
Understanding fuel economy units
Fuel economy can express distance per fuel volume, such as km/L or mpg, or fuel volume per distance, such as L/100 km. These directions are reciprocal: better efficiency gives a higher mpg or km/L value but a lower L/100 km value.
Where these units are used
Drivers use these conversions to compare vehicle specifications, estimate trip fuel, and interpret imported reviews. Real consumption changes with speed, traffic, weather, load, tires, terrain, and driving style.
Rounding and precision
One decimal place is usually adequate for vehicle comparisons. Keep more digits when estimating total trip fuel, then round to a practical volume. Published test-cycle numbers are standardized estimates, not guarantees.
Regional differences
US mpg and Imperial mpg use different gallons. The same numeric mpg therefore represents different efficiency. L/100 km is common in Europe and many other regions, while the US commonly uses mpg.
Common mistakes
Do not convert L/100 km with a simple multiplication factor; it requires division. Zero consumption is not a valid denominator, and US mpg must not be substituted for Imperial mpg.
USEFUL ANSWERS
Fuel Economy conversion FAQs
Why does lower L/100 km mean better economy?
It measures fuel consumed for a fixed distance, so using fewer liters is more efficient.
Are US mpg and Imperial mpg equal?
No. The Imperial gallon is larger, so the numeric mpg values differ for the same vehicle efficiency.
Why is zero invalid in a reciprocal conversion?
The reciprocal formula would divide by zero, which has no finite result.