What the rated figure actually measures

What the rated figure measures

The rated figure comes from a laboratory test done before the car goes on sale. The car sits on rollers and follows a fixed speed trace while the exhaust is collected, and fuel use is worked back from that. Nobody drove it down a road to get the number. Every car is measured the same way.

So the value of the rated figure is comparison, not absolute accuracy. Because the yardstick is the same, the statement that A is more efficient than B holds up. The statement that your car will do exactly that number does not. The test trace has no hard acceleration, no hills, no air conditioning, no winter warm up and no luggage.

In practice you will see less than the rated figure when traffic is heavy, when your trips are short, when it is cold, and when your tyres are soft. You may see more than it on a long steady motorway run. This site uses the rated figures as they are. We have no real world driving data, so we do not adjust them.

What is the number for then. It is for putting two cars on the same footing. Read the difference rather than the absolute amount. Because both sides came off the same yardstick, the direction of the gap tends to hold even when the exact money moves.

Combined, city and highway

The file carries three figures. The city figure comes from a low speed test full of stops and starts. The highway figure comes from a steadier, faster run. The combined figure mixes the two in a fixed ratio, and that is the one printed large on the label in the window.

Combustion cars usually do better on the highway figure than the city one, because there is no stopping and starting. Electric cars are often the other way round, because regenerative braking returns energy at low speed while air resistance grows at high speed. You can see that reversal in the data.

So the figure to use depends on where you drive. If your commute is stop and go, the city figure is closer to your life. If you cover long motorway trips, use the highway figure. If you are not sure, use the combined one. This site lets you pick any of the three and disables the ones the file does not carry for that car.

The grade is the combined figure sorted into bands, from grade 1 down to grade 5. The bands are drawn differently for different vehicle classes, so two cars with the same grade are not necessarily equally efficient. Reading the number itself is more precise.

How this site works

There is one source, the Korea Energy Agency vehicle fuel economy file published on the national open data portal. The original file sits in the repository and a build script reads it into the shape the screen uses. Nothing is scraped from other sites. The file is republished once a year and so is our copy.

The file sometimes carries the same name on several rows. Rows identical down to the last figure were merged into one. Rows with different figures were left separate, because the file has no model year column and we cannot tell which is which. The default is the row with the lowest combined figure. Leaning towards saving less does less harm when we are wrong, and you can switch rows on screen.

The file has no column for fuel type. So we do not know whether a car takes petrol, diesel or LPG. What we do know is what the agency states in its own note on the file: only electric and plug in hybrid cars carry a charge range, and economy is stated in km per litre. Cars whose grade column reads PHEV or FCEV are taken at their word.

When a model name happens to contain characters like diesel or LPi, that preselects the fuel. This is not a ruling about the car, only the fact that those characters are in the name, and the screen says so. When there is nothing in the name, petrol is preselected as our default rather than as a claim from the data. Either way you can change it.

Fuel prices are not fetched. There is no server here, and a live price baked into a static file would be wrong the moment it was baked. The default prices on screen are round numbers we chose and the screen says so. Type a price you know and everything recounts.

The arithmetic is one division and one multiplication. Distance divided by economy gives the fuel used, multiplied by the price gives the yearly cost. Plug in hybrids get a range rather than a single figure, because how much of the driving is electric is not in the data. When two ranges overlap we do not say which car is cheaper.

The cars you pick and the numbers you type never leave the browser. The server counts only how far a visit got: opened the page, picked a car, saw a result, copied a link, made a first input, along with the date, the language and a rough source label. Which cars you looked at is not recorded.

Counting a year of fuel

The arithmetic is easy. Yearly distance divided by economy gives the fuel used in a year, and that multiplied by the price per litre gives the yearly cost. A car doing 12km per litre over 15,000km uses 1,250 litres, which at 1,700 won a litre is 2,125,000 won.

The hard part is knowing your yearly distance. The most accurate way is to read the odometer and divide by how long you have owned the car. Failing that, work from your commute: one way distance times two, times the days a week you go in, times the weeks a year you go. This site counts 48 weeks and leaves out holidays and weekend driving.

For the price, today's forecourt figure is a good start, but you are multiplying it by a whole year, so something closer to the average of recent months lands nearer reality. Prices move, and one day's number times a year moves with them.

When you read the gap, look at direction and size rather than the exact money. A gap of a few hundred thousand won a year rarely flips when prices shift. A gap of a few tens of thousands can flip on driving style alone.

And fuel is only part of running a car. Insurance, road tax, servicing and depreciation are still there, and often larger. This site has no data for those and counts fuel only. Do not decide on a car from the fuel line alone.

Electric, plug in hybrid and hydrogen

Electric cars are rated in km per kilowatt hour. That is a different unit from km per litre, so the raw numbers cannot be compared with a combustion car. To compare them you have to turn both into money, which is what this site does. Distance divided by economy gives kilowatt hours, multiplied by your tariff gives the cost.

What you pay per kilowatt hour depends heavily on where you charge. Charging at home overnight and using a motorway rapid charger differ by several times. That is why the electricity price is yours to type in. The default is a round number, not your tariff.

A plug in hybrid runs on both. Its economy cell holds two figures separated by a slash, electric first in km per kilowatt hour and fuel second in km per litre. How much of your driving is electric is not in the data and differs by person, so this site gives a range instead of a single figure.

Short trips with a charge every night put a plug in hybrid near the low end of that range. Rarely charging puts it near the high end. You know where you sit, which is exactly why we do not decide it for you.

Hydrogen cars are rated in km per kilogram. In this file those are the rows whose grade column reads FCEV, all of them Nexo variants. Hydrogen prices differ by station, so type in one you know.

Range on a charge appears only for electric and plug in hybrid cars. It does not enter the fuel cost, but it tells you roughly how often you will be charging. It is also a test figure, and it shortens in winter.

Where people get stuck

If the car will not come up, the name in the file may differ from the one you know. Model names are filed as the maker declared them, often down to the trim and the wheel size. Try one short word, then start from the maker list. If it is still not there, the car is not in this file, and we will not invent it.

If one name shows several different figures, the file really does carry several rows for it. They look like model years or trim differences, but the file has no such column and we cannot confirm it. Your own car's rating is on its registration document and on the window label.

When the screen says not measured, that cell in the file was blank or zero. The agency notes that such values are not measured. Counting with zero would give zero or infinity, so we say there is no figure instead.

If a result looks wildly high or low, check the units. Electric economy is per kilowatt hour and that price has a different number of digits from a litre price. Putting a petrol price in the electricity box multiplies the answer several times over.

If your real fuel spend differs from the figure here, that is usually the gap between the rated figure and real driving described in the first section. If you know your own real economy, working it out by hand with that number beats this site.