How to use the Fuel Economy Converter
- Enter your fuel economy figure.
- Select its unit, taking care to distinguish US from imperial MPG.
- Read all four equivalents in the results.
- Remember that higher MPG is better while lower L/100 km is better.
- Use the MPG calculator to work the figure out from tank fills in the first place.
How the calculation works
Two families of measure exist and they are reciprocals of each other. MPG and km/L are efficiency measures — distance per unit fuel, where higher is better. Litres per 100 km is a consumption measure — fuel per unit distance, where lower is better. Converting between the families requires inverting the number, not scaling it, which is why the relationship is a curve rather than a straight line.
That curvature has a practical consequence usually called the MPG illusion. Improving from 15 to 20 MPG saves far more fuel over the same distance than improving from 40 to 50 MPG, even though the second gain looks larger. Over 10,000 miles, the first saves 167 gallons and the second only 50. Consumption units make this obvious immediately, which is one reason most of the world reports L/100 km.
The gallon problem compounds everything. An imperial gallon is 4.54609 L against the US 3.785 L, about 20% larger, so an identical car reads roughly 20% better in imperial MPG. A figure of 40 imperial MPG is only 33.3 US MPG. Any cross-market comparison must state which gallon applies, and manufacturer figures from official test cycles carry their own separate caveats about real-world accuracy.
km/L = US MPG × 0.4251437 = imperial MPG × 0.354006; L/100 km = 100 ÷ (km/L); US MPG = imperial MPG × 0.832674Source: NIST Handbook 44 Appendix C; UK Weights and Measures Act 1985; EPA fuel economy test procedures (40 CFR Part 600).
Worked example
A car is advertised at 6.5 L/100 km and you want to compare it with a US listing quoted in MPG.
- Enter 6.5 and select litres per 100 km.
- Invert to km/L: 100 ÷ 6.5 = 15.38 km/L.
- Convert to US MPG: 15.38 ÷ 0.4251437 = 36.2 MPG.
- Convert to imperial MPG: 15.38 ÷ 0.354006 = 43.4 MPG.
6.5 L/100 km is 36.2 US MPG or 43.4 imperial MPG — the same car, three numbers, and only the units differ.
Frequently asked questions
Is US MPG the same as UK MPG?+
No. Imperial MPG figures are about 20% higher for identical consumption because the imperial gallon is larger.
Why is L/100 km inverted relative to MPG?+
Because it measures fuel per distance rather than distance per fuel. Lower is better on that scale, and the two are reciprocals.
What is the MPG illusion?+
The fact that equal MPG gains are not equal fuel savings. Going from 15 to 20 MPG saves far more fuel than 40 to 50 MPG over the same mileage.
Why doesn't my car match its official figure?+
Official figures come from standardised test cycles. Cold starts, short trips, speed, load, tyre pressure and climate control all reduce real-world economy, often by 10 to 20%.
Last reviewed September 1, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.