How to Compute Fuel Efficiency

Fuel Economy Calculator
Fuel Used

Fuel efficiency is a ratio of distance to amount of fuel used. In the countries that use miles and gallons, fuel efficiency is measured in miles per gallon, MPG. In countries that use the metric system, fuel economy is recorded in kilometers per liter, KMPL or KPL. The higher the ratio, the more efficiently the vehicle uses gas.

Some countries use a third ratio, liters per 100 kilometers, abbreviated L/100km. This ratio is properly called the fuel consumption. The lower the value, the more efficient the vehicle.

You can compute fuel efficiency or consumption as long as you know the distance traveled and the total amount of fuel used on the journey. To get the most accurate reading, begin with a full tank of gasoline. Then set the trip odometer to zero and start driving around as you normally do during the day.

When you get bored of driving around, pull up to the gas station pump and note the total distance traveled. Call this number D. Next, refill your tank so that it is completely full and record the amount of gas you put in your vehicle. Call this number F. The miles per gallon ratio is given by the formula

MPG = D/F,

where D is in miles and F is in gallons. The km per liter ratio is given by the identical equation


except D is in kilometers and F is in liters. To convert between MPG and KMPL with US gallons, use the equations

KMPL = 0.425143707 × MPG      Ex: 15.6 KMPL = 36.7 MPG
MPG = 2.352145836 × KMPL      Ex: 30 MPG = 12.75 KMPL

To convert between MPG and KMPL with Imperial gallons, use the equations

KMPL = 0.35400604 × MPG
MPG = 2.824810534 × KMPL

Liters per 100 Kilometers

Many countries measure fuel consumption in liters used per 100 km driven. To calculate L/100km, simply divide the number 100 by KMPL.

Example: Suppose a car uses 187 liters of fuel to drive 2303 km. The fuel economy is 2303/187 = 12.3155 KMPL. The fuel consumption is 100/12.3155 = 8.12 L/100km.

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