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Automotive

MPG Calculator

Fuel economy in mpg, L/100 km and km per litre.

Your measurementsEvery assumption editable01
Fuel economyCalculated live02

Result

25.8mpg

25.8 miles per gallon

Distance
320 miles
Fuel used
12.4 gal
Fuel economy
25.81 mpg
In L/100 km
9.11
In km per litre
10.97

Fill to the same click on the pump at both ends of the tank. Brim-filling one time and stopping at the first cutoff the next introduces more error than anything you are trying to measure.

One tank is a data point, not a trend. Weather, traffic, tyre pressure and fuel blend all move economy by several per cent, so average across several tanks before concluding anything.

US and imperial gallons differ by about twenty per cent, which is why the same car is quoted at very different mpg figures either side of the Atlantic. This uses US gallons.

Distance divided by fuel used. The arithmetic is trivial; the accuracy depends entirely on filling to the same point on the pump at both ends of the tank.

Why use this tool?What it does differently03

Why use this tool?

Three units

mpg, L/100 km and km per litre, for comparing against figures quoted either side of the Atlantic.

Method guidance

The commonest error is inconsistent filling, not arithmetic.

One tank is not a trend

Weather, traffic and tyre pressure move economy by several per cent.

US gallons, stated

US and imperial gallons differ by about twenty per cent, which explains a lot of confusing comparisons.

How this worksThe method04

How this mpg calculator works

Miles travelled divided by gallons consumed. The conversions follow: 235.215 divided by mpg gives litres per hundred kilometres, and multiplying mpg by 0.425144 gives kilometres per litre.

Accuracy lives in the method rather than the maths. Fill to the same click of the pump at both ends of the measured tank — brimming one time and stopping at the first cutoff the next introduces more error than anything you are trying to detect.

This uses US gallons. Imperial gallons are about twenty per cent larger, which is why the same car is quoted at very different mpg figures in the US and the UK.

How to use itStep by step05

How to use it

  1. Step 1: Fill and reset

    Fill to the first pump cutoff and zero the trip meter.

  2. Step 2: Drive a full tank

    A longer measured distance dilutes the error in the fill.

  3. Step 3: Refill the same way

    Same pump behaviour, same cutoff. Record the gallons added.

  4. Step 4: Average several tanks

    One tank is a data point; three or four is a trend.

Example usageWorked figures06

Example usage

A typical tank
320 miles on 12.4 gallons is 25.81 mpg, or 9.11 L/100 km.
A truck
280 miles on 20.5 gallons is 13.66 mpg — 17.22 L/100 km.
A hybrid
540 miles on 10.2 gallons is 52.94 mpg, or 4.44 L/100 km.
Frequently asked questionsCommon questions07

Frequently asked questions

How do I calculate MPG?

Divide miles travelled by gallons of fuel used. Fill the tank, zero the trip meter, drive, refill and divide the trip distance by the gallons added.

Why does my calculated MPG differ from the dashboard?

Dashboard displays estimate from injector duty cycle rather than measuring fuel, and most read slightly optimistic. A measured tank is the more reliable figure.

How do I convert MPG to L/100 km?

Divide 235.215 by the mpg figure. Note the relationship is inverse — lower L/100 km is better, higher mpg is better.

Are US and imperial gallons the same?

No. An imperial gallon is about twenty per cent larger, so the same car shows a noticeably higher mpg figure quoted in imperial.

How many tanks should I measure?

At least three or four. Weather, traffic, tyre pressure and fuel blend each move economy by several per cent, so a single tank tells you very little.

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