Use the free Fuel Efficiency Calculator below: your data never leaves your browser — all work happens on your device, and nothing is uploaded to any server.
Two Ways to Say the Same Thing
Fuel efficiency is reported in two opposite ways around the world, and that causes endless confusion. Litres per 100 kilometres tells you how much fuel you use to cover a fixed distance: lower is better. Miles per gallon tells you how far you go on a fixed amount of fuel: higher is better. This calculator gives you both figures from one measurement, plus kilometres per litre, so you can compare any vehicle, any review, and any country without converting by hand.
The confusion is more than academic. A car advertised at 5 litres per 100 kilometres sounds efficient, and it is, but many drivers cannot picture what 5 litres per 100 kilometres means for a tank or a trip. The same car achieves about 47 miles per gallon, which is easier to picture for drivers used to gallons, but harder for drivers used to litres. Giving both numbers removes the translation barrier and lets you judge the car, not the unit.
How the Calculator Works
Enter how far you drove and how much fuel you used, in whatever units you measured them in. Distance can be kilometres or miles, fuel can be litres or US gallons, and the calculator converts both to a common base before dividing. From litres and kilometres it works out litres per 100 kilometres directly, then converts that into miles per gallon and kilometres per litre. If you enter a fuel price, it also shows total fuel cost for the measured trip and cost per kilometre.
A Worked Example
Test the same trip in imperial units to check the conversion. Driving 250 miles on 7.4 US gallons also gives about 33.8 miles per gallon, matching the metric result within rounding. If your two methods disagree by more than a small rounding amount, check whether you used US gallons or imperial gallons, because an imperial gallon is about 20 percent larger and will inflate the miles-per-gallon figure if mixed up with US gallons.
Measuring Efficiency Honestly
One tank is not enough for a reliable figure. Fill the tank fully, reset the trip meter, drive normally, and fill fully again, recording litres added and kilometres driven. That gives one measurement, but a single measurement can be skewed by a partial fill, a different pump cut-off, or an unusual week of driving. Average over three to five tanks for a figure you can trust, and keep city and motorway driving separate if you want to predict specific trips.
Also note what was different during the measurement. A roof box, a full load, winter tyres, or mostly short trips will all worsen the figure, while gentle motorway cruising will improve it. An efficiency figure without context is just a number. An efficiency figure with context, such as 7 litres per 100 kilometres in mixed driving over five tanks, is a planning tool you can actually use.
What Efficiency Does Not Tell You
Efficiency tells you fuel per distance, but it does not tell you total cost of ownership. A very efficient car that costs far more to buy, insure, or maintain may cost more overall than a thirstier car that is cheap to run in other ways. Fuel is only one part of motoring cost, alongside depreciation, insurance, tax, servicing, tyres, and repairs. Use efficiency to compare fuel costs fairly, then add the other costs before choosing a vehicle.
Efficiency also does not measure emissions directly, though the two are closely linked for petrol and diesel cars: burning less fuel generally means emitting less carbon dioxide per kilometre. For a fuller environmental picture, consider the fuel type, the vehicle size, and how much you drive, because a very efficient car driven 30,000 kilometres a year can emit more in total than a thirstier car driven 5,000 kilometres a year.
Frequently Asked Questions
Why do US and UK miles per gallon differ? Because a US gallon is 3.785 litres while an imperial gallon is 4.546 litres. The same car shows a higher miles-per-gallon figure in the UK than in the US, purely from the gallon size. This calculator uses US gallons, so state that when comparing with UK figures.
Is a lower litres per 100 kilometres always better? Yes, for fuel use per distance. Lower means less fuel to cover the same road, which means lower fuel cost and usually lower emissions per kilometre.
Can I use this for motorcycles or trucks? Yes. The maths is the same for any vehicle that burns liquid fuel, as long as distance and fuel are measured in consistent units over the same trips.
Use the tool above with your own measured tanks, and compare vehicles on both scales at once.
Disclaimer: The results shown by this tool are ESTIMATES only. They are for education and planning, not financial or tax advice. Your real amounts will differ because rates, taxes, fees and prices change. Talk to a qualified financial or tax professional before you make a money decision.
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