What Is Fuel Economy?
James drives a delivery van for a small business in Phoenix. At the end of a 420-mile route, he fills up the tank and notes that he pumped 15.2 gallons. His fuel economy is 420 divided by 15.2, which equals 27.6 miles per gallon. That single number tells him how efficiently his van converts fuel into distance, and it lets him compare vehicles, track maintenance needs, and budget for fuel costs. Fuel economy is the relationship between distance traveled and fuel consumed, and it is one of the most useful metrics any driver can track.
Fuel economy is expressed in two common ways. In the United States, it is measured in MPG (miles per gallon), where a higher number is better. In most other countries, it is measured in L/100km (liters per 100 kilometers), where a lower number is better. This calculator shows both values simultaneously so you can compare regardless of which system you use. To track MPG from odometer readings over multiple fill-ups, see our Gas Mileage Calculator.
What This Calculator Does
This fuel economy calculator takes the distance you traveled and the amount of fuel you used, then computes both MPG and L/100km. It supports both US and metric input units.
- Inputs: Distance traveled and fuel used, with a unit system toggle
- Outputs: MPG, L/100km, and conversion formulas
How the Calculation Works
MPG = Distance (miles) / Fuel (gallons)
L/100km = Fuel (liters) / Distance (km) x 100
Conversion: MPG = 235.215 / L/100km
The conversion factor 235.215 comes from the relationship between miles, kilometers, gallons, and liters. One mile is 1.60934 kilometers and one gallon is 3.78541 liters. The factor 100 x 3.78541 / 1.60934 = 235.215 lets you convert between the two systems directly. For example, 30 MPG equals 235.215 / 30 = 7.84 L/100km.
How to Use the Calculator
- Select your unit system (US for miles and gallons, metric for km and liters)
- Enter the distance you traveled
- Enter the total amount of fuel used for that distance
- Review your fuel economy in both MPG and L/100km
Example Calculation
Example: A driver in Chicago travels 385 miles and uses 14.8 gallons of gas. MPG = 385 / 14.8 = 26.01. L/100km = 235.215 / 26.01 = 9.04. So the vehicle achieves 26.0 MPG, which is equivalent to 9.0 L/100km. If the driver wants to estimate the cost of this trip, they can multiply 14.8 gallons by the gas price using our Fuel Cost Calculator.
Common Mistakes to Avoid
- Not filling the tank completely: To get an accurate fuel economy reading, fill the tank to the same level each time. Partial fills introduce significant error because you cannot know exactly how much fuel was consumed.
- Mixing units: If you enter distance in miles but fuel in liters, the MPG result will be wrong. Always match the unit system to your inputs, or convert manually first.
- Comparing a single trip: Fuel economy varies with driving conditions. A single trip may not represent your average. Track fuel economy over 3 to 5 fill-ups for a reliable figure.
Limitations of This Calculator
This calculator computes fuel economy from a single distance and fuel amount. It does not track multiple fill-ups, account for tank-to-tank variation, or estimate costs. For odometer-based tracking across multiple fill-ups, use our Gas Mileage Calculator. For tire size comparisons that affect speedometer and fuel economy readings, see our Tire Size Calculator.