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MPG Calculator

Work out your car's real fuel economy in US miles per gallon from the miles you drove and the gallons you used.

miles

Miles driven since you last filled the tank.

US gallons

US gallons it took to refill the tank. Not imperial (UK) gallons.

Results

Fuel economy (US MPG)

30 MPG (US)

Miles per US gallon. Higher is better.

Fuel consumption
7.84 L/100 km

Fuel economy 30 miles per US gallon. 7.84 litres per 100 kilometres.

Formula

Fuel economy (US MPG)
= miles driven ÷ US gallons used
Fuel consumption (L/100 km)
= 235.214583 ÷ US MPG

235.214583 is 100 × 3.785411784 litres per US gallon ÷ 1.609344 km per mile, so dividing it by MPG converts miles per gallon into litres per 100 km in one step.

Worked example

You filled the tank, drove 300 miles, and it took 10 US gallons to fill it up again.

  1. Step 1: Fuel economy

    300 ÷ 10 = 30 MPG (US)

  2. Step 2: Convert to L/100 km

    235.214583 ÷ 30 = 7.84 L/100 km

The car achieved 30 miles per US gallon, which is 7.84 litres per 100 kilometres.

How to measure your MPG

The fill-up method measures the fuel your car actually used, so it reflects your own driving:

  • Fill the tank until the pump clicks off, then reset the trip odometer.
  • Drive as you normally would. A longer distance gives a more reliable figure.
  • Fill up again until the pump clicks off, and note the gallons on the pump.
  • Enter the miles on the trip odometer and those gallons into the calculator.

This calculator uses US gallons

A US gallon is 3.785411784 litres. The UK (imperial) gallon is 4.54609 litres, about 20% larger. If you enter imperial gallons here, the result will not be a correct US MPG figure, and it cannot be compared directly with UK MPG ratings either.

Getting a more reliable figure

Pumps do not click off at exactly the same level every time, so one short measurement can be a little off. Adding up the miles and gallons over several fill-ups and entering the totals evens this out.

Real-world MPG depends on speed, traffic, load, weather, tyre pressures and driving style, so it is normal for it to vary from one tank to the next and to differ from the official rating.

Frequently asked questions

How do I calculate MPG?

Divide the miles you drove by the gallons of fuel you used. For example, 300 miles on 10 US gallons is 300 ÷ 10 = 30 MPG.

Is this US MPG or UK MPG?

US MPG: miles per US gallon of 3.785411784 litres. UK MPG uses the imperial gallon of 4.54609 litres, so the same car has a higher UK MPG figure than US MPG figure.

How do I convert my MPG to L/100 km?

Divide 235.214583 by your US MPG. The calculator shows this conversion next to your MPG result, and the MPG to L/100 km converter does it for any figure.

Is a higher MPG better?

Yes. MPG is how far the car goes on one gallon, so a higher number means it uses less fuel for the same distance.

Why is my MPG different from the dashboard reading?

The dashboard estimates fuel use from engine data, while the fill-up method measures the fuel you actually bought. The two can differ slightly, and the dashboard may cover a different stretch of driving.

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