Fuel Efficiency Converter
Convert between MPG, L/100km, km/L and Imperial MPG. Calculate fuel costs and CO2 emissions for any trip distance.
Fuel Efficiency Converter
Calculator
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Formula: L/100km = 235.215 / MPG(US) | MPG(UK) = MPG(US) x 1.20095
Worked example — 7.84 L/100km | 12.76 km/L | 36.03 MPG UK | $58.33 fuel cost
Formula
L/100km = 235.215 / MPG(US) | MPG(UK) = MPG(US) x 1.20095
Conversion uses the gallon-to-liter ratios: 1 US gallon = 3.78541 L, 1 Imperial gallon = 4.54609 L, 1 mile = 1.60934 km. Fuel cost = (Distance / Efficiency) x Price. CO2 = Liters consumed x 2.31 kg/L.
Worked Examples
Example 1: Convert 30 MPG (US) to Other Units
Problem:A car is rated at 30 MPG (US). Convert to L/100km, km/L, and MPG (UK). Calculate fuel cost for a 500-mile trip at $3.50/gallon.
Solution:L/100km = 235.215 / 30 = 7.84 L/100km km/L = 100 / 7.84 = 12.76 km/L MPG (UK) = 282.481 / 7.84 = 36.03 MPG (UK) Fuel for 500 miles = 500 / 30 = 16.67 gallons Cost = 16.67 x $3.50 = $58.33 CO2 = 16.67 x 3.785 x 2.31 = 145.8 kg
Result:7.84 L/100km | 12.76 km/L | 36.03 MPG UK | $58.33 fuel cost
Example 2: European Car L/100km to US MPG
Problem:A European car is rated at 5.5 L/100km. Convert to US MPG and calculate fuel cost for 1000 km at $1.60/liter.
Solution:MPG (US) = 235.215 / 5.5 = 42.77 MPG MPG (UK) = 282.481 / 5.5 = 51.36 MPG km/L = 100 / 5.5 = 18.18 km/L Fuel for 1000 km = (5.5 / 100) x 1000 = 55 liters Cost = 55 x $1.60 = $88.00 CO2 = 55 x 2.31 = 127.1 kg
Result:42.77 MPG US | 51.36 MPG UK | 55 liters needed | $88.00 cost
Frequently Asked Questions
What is the difference between MPG (US) and MPG (Imperial/UK)?
The primary difference lies in the size of the gallon used. A US gallon equals 3.78541 liters, while an Imperial (UK) gallon equals 4.54609 liters, roughly 20 percent larger. This means a car rated at 30 MPG in the US system would be rated at approximately 36 MPG in the Imperial system for the same actual fuel consumption. This discrepancy often causes confusion when comparing vehicles across markets. When reading European or Australian car reviews that cite MPG figures, they typically use Imperial gallons. Japanese and most other markets use kilometers per liter (km/L) or liters per 100 kilometers (L/100km) instead, avoiding the gallon confusion entirely. Always verify which gallon standard is being referenced before comparing fuel economy figures.
How does L/100km relate to MPG and which is more intuitive?
L/100km (liters per 100 kilometers) is an inverse metric compared to MPG (miles per gallon). While MPG tells you how far you can travel on a fixed amount of fuel, L/100km tells you how much fuel you need for a fixed distance. The conversion formula is L/100km = 235.215 / MPG(US). Many experts argue that L/100km is actually more intuitive for comparing fuel savings because it scales linearly with fuel cost. For example, improving from 10 to 20 MPG saves much more fuel than improving from 30 to 40 MPG, but the MPG numbers make both look like equal improvements of 10 MPG. In L/100km, those same changes are 23.5 to 11.8 versus 7.8 to 5.9 liters, making the diminishing returns immediately obvious.
What factors affect a vehicle's real-world fuel efficiency?
Real-world fuel efficiency can differ significantly from manufacturer ratings due to numerous factors. Driving behavior is the largest variable, with aggressive acceleration and hard braking potentially reducing efficiency by 15 to 33 percent compared to smooth driving. Speed matters enormously as aerodynamic drag increases with the square of velocity, meaning highway driving above 65 mph significantly hurts efficiency. Tire pressure being just 1 psi below recommended can reduce efficiency by 0.2 percent. Extra weight costs roughly 1 percent efficiency per 100 extra pounds. Air conditioning can reduce efficiency by 5 to 25 percent depending on conditions. Cold weather reduces efficiency by 15 to 24 percent for short trips as the engine and transmission need time to reach optimal operating temperature.
How do I calculate fuel cost for a road trip?
To calculate fuel cost for a road trip, you need three pieces of information: total distance, your vehicle's fuel efficiency, and the current fuel price. The formula is straightforward: Fuel Cost = (Distance / Fuel Efficiency) x Price per unit. For example, a 500-mile trip in a car getting 25 MPG with gas at $3.50 per gallon costs (500 / 25) x 3.50 = $70.00. In metric units, a 800 km trip at 8 L/100km with fuel at $1.50/liter costs (800 / 100) x 8 x 1.50 = $96.00. For more accuracy, account for city versus highway driving proportions, elevation changes, and weather conditions. Mountain driving and heavy headwinds can increase consumption by 10 to 20 percent above rated efficiency figures.
What is the CO2 emissions relationship to fuel consumption?
Carbon dioxide emissions are directly proportional to fuel consumed because CO2 is a fundamental combustion byproduct. Each liter of gasoline burned produces approximately 2.31 kilograms (5.09 pounds) of CO2, while each liter of diesel produces about 2.68 kilograms (5.91 pounds). At 2.31 kg per litre, one US gallon of 3.78541 litres carries about 8.74 kg (19.3 lbs) of CO2. This means a car averaging 30 MPG emits roughly 291 grams of CO2 per mile, while a 20 MPG vehicle emits about 437 grams per mile. Over 12,000 miles of annual driving, that is around 3.50 metric tons of CO2 versus 5.25 metric tons. Understanding this direct relationship helps quantify the environmental impact of vehicle choice and driving habits.
Where does the constant 235.215 in the MPG to L/100km conversion come from?
It is built from two unit definitions rather than being an empirical figure. One US gallon is 3.78541 litres and one mile is 1.60934 kilometres, so kilometres per litre equals MPG × 1.60934 ÷ 3.78541 = MPG × 0.425144. Inverting that and scaling to 100 km gives 100 ÷ 0.425144 = 235.215, so L/100km = 235.215 ÷ MPG. The Imperial constant is derived the same way with the 4.54609 litre gallon: 100 × 4.54609 ÷ 1.60934 = 282.481, which is why this converter carries both numbers.
How do kilometres per litre and litres per 100 kilometres relate as reciprocal fuel-economy units?
Kilometres per litre is a distance-per-fuel measure and litres per 100 kilometres is a fuel-per-distance measure, so they are reciprocals scaled by 100: litres per 100 km equals 100 divided by kilometres per litre, and the reverse holds as well. A vehicle achieving 12.5 kilometres per litre consumes 100 divided by 12.5, or 8.0 litres per 100 kilometres. Because the relationship is inverse, equal steps in one are not equal steps in the other, which matters when comparing fuel economy quoted in Japanese kilometres per litre against European litres per 100 kilometres.
How does MPGe let you compare an electric vehicle against a gasoline one?
MPGe puts electricity on a gasoline-equivalent energy scale by treating 33.7 kilowatt-hours as the energy content of one gallon of gasoline. Multiply an EV's efficiency in miles per kWh by 33.7 to get MPGe: 3.5 mi/kWh × 33.7 ≈ 118 MPGe. It is an energy comparison, not a cost or emissions comparison, because a kWh and a gallon are priced very differently and grid carbon intensity varies by region. Use MPGe to compare how efficiently two vehicles convert energy into distance, and cost per mile to compare what you will actually pay.
What does a combined fuel economy rating mean and when should you use the city or highway figure instead?
A combined rating is a weighted blend of the city and highway test cycles — in the US EPA scheme, 55 percent city and 45 percent highway — meant to represent an average driver. It is the right input when your driving genuinely mixes both. If your mileage is overwhelmingly one type, use the matching cycle figure: highway for a long-distance commuter, city for stop-start urban use. The distinction is sharpest for hybrids, which often post a higher city than highway figure because regenerative braking recovers energy during frequent stops.
How do you measure your own fuel economy with the tank-to-tank method?
Fill the tank to the same cut-off point, note the odometer, drive normally, then refill to that same point. Divide the distance covered by the fuel added: miles ÷ US gallons gives MPG, or litres added × 100 ÷ kilometres gives L/100km. The key is consistency at the pump, since the auto shut-off level varies with nozzle angle and station. Trip-computer readouts are model-based estimates rather than direct measurements, so averaging three or four consecutive tank-to-tank fills is more reliable than a single one.
References
Reviewed for accuracy by Daniel Agrici, Founder & Lead Developer · Editorial policy
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