Free Online Fuel Efficiency Converter
Convert MPG, km/L, and L/100km values.
Optional comparison
| Unit | This vehicle | Comparison |
|---|---|---|
| US MPG | — | — |
| UK MPG | — | — |
| km/L | — | — |
| L/100 km | — | — |
About the Fuel Efficiency Converter
Fuel efficiency is written differently around the world, and the numbers are not directly comparable. US miles per gallon uses a smaller gallon than the UK imperial gallon, kilometers per liter measures distance per unit of fuel, and liters per 100 kilometers measures fuel per distance in the opposite direction. This converter translates one value into all four formats at once, so you can compare a rental car, a used vehicle listing, a government label or a trip computer readout without guessing.
Why US and UK MPG differ
A US gallon is about 3.785 liters, while an imperial gallon is about 4.546 liters. The same car therefore travels more miles on an imperial gallon, so the UK MPG figure is always larger than the US figure for the same real consumption. The conversion factor is roughly 1.201, meaning 30 US MPG is about 36 UK MPG. Confusing the two can make a vehicle look around 20 percent more efficient than it is, especially in classified ads that do not state which gallon they use.
Distance per fuel versus fuel per distance
MPG and km/L are distance-per-fuel measures: larger numbers are better. L/100 km is a fuel-per-distance measure: smaller numbers are better. This inversion makes mental conversion tricky. The calculator uses km/L as an internal bridge, converting the input to kilometers per liter, then deriving the other three values. L/100 km is simply 100 divided by km/L. Because the relationship is nonlinear, a change from 5 to 6 L/100 km saves more fuel than a change from 15 to 16 L/100 km, even though both look like one unit.
Cost and comparison features
If you enter a fuel price per liter, the tool estimates the cost of driving 100 kilometers at the selected efficiency. This is useful for comparing a fuel-efficient car with a cheaper but thirstier one, or for estimating running costs on a long trip. The optional comparison field accepts a second efficiency in any of the four units and reports the percentage difference. The table repeats the comparison in every unit, so you can see the same result in the format used by the buyer, the seller or the fuel label.
Practical limits
Official efficiency figures come from standardized tests and rarely match real driving. Cold weather, short trips, air conditioning, roof racks, aggressive acceleration, traffic and tire pressure all move consumption in the wrong direction. Use the converter for comparison and planning, then treat real-world economy as a range rather than a single number. Keep units consistent when comparing vehicles: a US MPG figure must be compared with another US MPG figure, not with a UK MPG value.
Frequently asked questions
Which number is better, higher or lower?
For US MPG, UK MPG and km/L, higher is better. For L/100 km, lower is better because it measures fuel consumed rather than distance traveled. The comparison message accounts for this direction automatically.
Can I convert a value without entering a fuel price?
Yes. The four main conversions and the comparison table work without a price. The cost per 100 km card stays blank until a price is entered.
Why does doubling MPG not halve fuel use?
It does for a fixed distance: doubling distance per fuel halves the fuel used. But adding a fixed 1 MPG to a low-MPG vehicle saves far more fuel than adding 1 MPG to a high-MPG vehicle. That is why L/100 km is often a fairer comparison of real fuel savings.