Fuel Economy Calculator (Tank-Full Method)

＋More features: compare with the catalogue figure, compare two cars over a year

What percentage of the catalogue figure are you getting?

Checks how your real fuel economy compares with the catalogue figure (WLTC mode and similar). Around 70–80% is the usual range.

Real fuel economy (km/L) Filled in automatically if you calculated with the form above Catalogue fuel economy (km/L)

Two cars compared: yearly fuel cost difference (is it worth changing car?)

From your current car (real fuel economy), the fuel economy of the car you are considering and your yearly mileage, this shows how much your fuel bill would change in a year.

Fuel economy of your current car (km/L) Fuel economy of the car you are considering (km/L) Yearly distance driven (km) Mostly commuting and shopping is typically 5,000–10,000 km Fuel price (yen per litre) You can leave this out if you already entered it in the form above

What this tool tells you

Enter the distance you drove since your last fill-up and the amount of fuel you put in, and you get your real fuel economy in km/L — the unit used on Japanese car catalogues and at Japanese petrol stations. Add the price per litre and it also shows the cost of the fill-up and the fuel cost per kilometre. Handy for checking the health of your car or keeping a fuel log.

How to use

Fill the tank right up every time you refuel, and reset the trip meter.
At the next fill-up, enter the distance you drove and the number of litres that went in.
As soon as both are entered, your real fuel economy appears just below — no button needed. Add the price per litre to get the costs too.

How it is calculated

Fuel economy (km/L) = distance driven (km) ÷ fuel added (L) (tank-full method)
The tank-full method treats the fuel you add now as exactly the fuel used between one full tank and the next. It is the most reliable way to measure real-world fuel economy.
It also shows the European style L/100km = 100 ÷ fuel economy (km/L) (the litres needed to drive 100 km).
Example: 400 km driven on 25 L → 400 ÷ 25 = 16.0 km/L (6.25 L/100km). At 170 yen per litre, the fill-up costs 4,250 yen and driving costs about 10.6 yen per km.

Sources and assumptions

Arithmetic based on distance, volume and unit price.
For how the catalogue figure (WLTC mode) is measured, see the Ministry of Land, Infrastructure, Transport and Tourism (Japan), “Fuel economy performance of motor vehicles” (in Japanese).

Assumptions and notes

With the tank-full method, the point at which the pump nozzle clicks off varies, so a single measurement carries some error. Averaging several fill-ups gives a more accurate picture.
Real fuel economy is typically 20–30% below the catalogue figure (WLTC mode). Air conditioning, traffic jams and lots of short trips make it worse.

Frequently asked questions

My real fuel economy is much worse than the catalogue figure. Is something broken?

The catalogue value (WLTC mode) is measured on a fixed driving pattern, and on real roads it is normal to come out 20–30% lower. But if your fuel economy dropped suddenly, it is worth checking things like tyre pressure and engine oil.

What is L/100km?

It means “how many litres you use to drive 100 km”, the common way of quoting fuel economy in Europe. It works the opposite way to km/L: the smaller the number, the more efficient the car. 16 km/L equals 6.25 L/100km.

Can I use this for a hybrid or a kei car?

Yes. The calculation is the same for any petrol-powered car, whatever the model. (Fully electric vehicles use a different measure, km/kWh.)