β½ Fuel Efficiency Converter
Convert MPG, L/100km, and km/L, plus trip cost estimates.
Fuel Efficiency
How to Use This Converter
Enter a fuel economy figure in any unit to see it in all the others. Add a trip distance and fuel price to estimate what the trip costs in each car you compare.
Enter the fuel economy number and pick its unit: US MPG, UK MPG, L/100km, or km/L.
Read all four units. Watch the gallon: UK MPG runs about 20 percent higher than US MPG for the same car.
Optionally enter a trip distance and fuel price to compare trip costs.
Compare two cars by converting both to L/100km, where the math is linear.
The Conversion Formulas
The magic number 235.214583 comes from converting miles to kilometers and gallons to liters: 100 km / 1 mile-in-km, times liters per US gallon. UK MPG uses the imperial gallon (4.54609 L versus 3.78541 L), so multiply US MPG by 1.20095. The L/100km scale has a real advantage: it is linear in fuel use, so halving your L/100km halves your fuel bill, while MPG gains have diminishing returns (going 15 to 20 MPG saves more fuel than 40 to 50).
Worked Example
Car A: 30 US MPG. L/100km = 235.214583 / 30 = 7.84. km/L = 100 / 7.84 = 12.75. UK MPG = 30 x 1.20095 = 36.03. Car B: 5.5 L/100km = 42.77 US MPG. Over a 300-mile trip at $3.50/gal: Car A uses 300/30 = 10 gallons = $35.00. Car B uses 300/42.77 = 7.01 gallons = $24.55. The efficient car saves $10.45 on one trip, about $540 over 15,000 miles a year.
Why Ratings Disagree Across Borders
Even after converting units, US EPA and EU WLTP test cycles measure differently: different speeds, temperatures, and accessory loads. A car rated 6.0 L/100km in Europe might rate 35 MPG (6.7 L/100km) in the US. Real-world driving adds another 10 to 20 percent over lab figures for most drivers. Use converted ratings to compare cars within one system, and treat cross-border comparisons as approximate.
Frequently Asked Questions
What is the difference between US and UK MPG?
The gallons differ. A US gallon is 3.78541 liters; an imperial (UK) gallon is 4.54609 liters, about 20 percent larger. So 30 US MPG equals 36.03 UK MPG: the same car, the same fuel, a bigger-sounding number in Britain. Always check which gallon a figure uses.
Is L/100km better when lower?
Yes. L/100km measures consumption (fuel per distance), so lower is better, unlike MPG where higher is better. That makes comparisons easier: a car using 5 L/100km instead of 10 literally uses half the fuel, while 50 MPG versus 25 MPG needs a moment of thought to see the same thing.
How do I compare EU and US fuel economy?
Convert both to the same unit. EU figures are published in L/100km; US figures in MPG. Divide 235.214583 by the US MPG to get L/100km, or divide 235.214583 by the L/100km figure to get US MPG. Note that EU (WLTP) and US (EPA) test cycles differ, so the numbers are not perfectly comparable even after conversion.
How much does it cost to drive 100 miles?
Divide the trip distance by your MPG to get gallons used, then multiply by the fuel price. At 30 MPG and $3.50 per gallon: 100 / 30 = 3.33 gallons, x $3.50 = $11.67. The trip estimator in this converter does it for any distance and price.
What is a good fuel economy in 2026?
For a gasoline car, 30+ US MPG (under 7.8 L/100km) is good, 40+ is excellent and usually hybrid territory. The average new US vehicle sits around 26 MPG. Electric cars are rated in MPGe, a different scale where 100+ MPGe is common.