Vehicle Mileage Calculator - Calculate Fuel Efficiency

Use the Vehicle Mileage Calculator to find your car's fuel efficiency (MPG or L/100km), total fuel cost, and average trip speed.

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What it is and when to use it

Fuel economy tells you how far a vehicle travels on a given amount of fuel. Tracking it shows whether a car is performing normally, helps compare vehicles or driving routes, and is often the first sign of a maintenance problem such as low tyre pressure or a failing sensor. This calculator takes the distance you drove and the fuel used and returns your fuel economy in either miles per gallon or litres per 100 kilometres.

Add a fuel price and it also computes the total cost of the trip and the cost per mile or kilometre, which is handy for budgeting a road trip or splitting costs with passengers. Add the trip time and it computes your average speed. Choose the unit system first, then enter distance and fuel in matching units.

The formula and how it works

The calculator uses direct ratios of the numbers you enter:

  • US mode: MPG = distance in miles / fuel in gallons.
  • Metric mode: L/100 km = fuel in litres / distance in km x 100, and km per litre = distance / fuel.
  • Total fuel cost = fuel used x price per unit.
  • Cost per mile or km = total fuel cost / distance.
  • Average speed = distance / trip hours.

Average speed includes any time spent stopped, because it divides by the full trip duration you enter.

Worked example

Say you drove 300 miles, used 12 gallons, paid $3.50 per gallon, and the trip took 5 hours. Fuel economy is 300 / 12 = 25.00 MPG. Total fuel cost is 12 x 3.50 = $42.00, and the cost per mile is 42 / 300 = $0.140.

Average speed is 300 / 5 = 60.0 mph. In metric mode, entering 300 km and 12 L gives 12 / 300 x 100 = 4.00 L/100 km, which is the same as 25.00 km per litre. Converting the US result for reference, 25 MPG is about 9.41 L/100 km, since 235.21 / 25 = 9.41. The calculator displays the values above for those inputs.

Common mistakes and how to interpret the result

  • Mixing units: entering miles with litres, or kilometres with gallons, gives a meaningless figure. Keep the unit system consistent.
  • Relying on a partial fill: if the tank was not filled to the same level both times, the fuel figure is wrong. Use full fill-ups for accuracy.
  • Judging from one tank: mileage swings with weather, traffic and load, so use an average over several tanks.
  • Reading L/100 km backwards: a lower number means better economy, the opposite of MPG.

Frequently Asked Questions

How do I measure mileage accurately?
Fill the tank completely, reset the trip meter, drive normally, then refill at the same pump if possible and note the gallons or litres added and the distance shown. Use that fill-up quantity as the fuel used. A single tank can be off by a few percent because of pump shut-off differences, so averaging several fill-ups gives a more reliable figure.
How do MPG and L/100 km relate?
They are inverse measures. MPG is distance per volume, so higher is better, while L/100 km is volume per distance, so lower is better. Using US units, L/100 km = 235.21 / MPG, so 25 MPG is about 9.41 L/100 km. Note that a US gallon is 3.785 litres and an imperial gallon is 4.546 litres, so UK MPG figures are not directly comparable.
Are the optional fields required?
No. Only distance and fuel are needed for fuel economy. Add a price per unit of fuel to see the total fuel cost and the cost per mile or kilometre, and add a trip duration to see the average speed. Leave them blank to skip those lines.
Why does my mileage differ from the car's display?
Onboard computers estimate consumption from injector data and often read a little optimistic, and odometers and speedometers have small calibration errors. Driving style, tyre pressure, load, weather, and the mix of city and highway driving also change fuel economy noticeably from tank to tank.

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