Fuel Efficiency Calculator

How fuel efficiency is calculated

Fuel efficiency in km/L is distance driven divided by fuel used. If you covered 450 km and the tank took 30 L to refill, the result is 450 / 30 = 15 km/L. The European way of writing the same fact, L/100km, isfuel used divided by distance, times 100: 30 / 450 x 100 is 6.67 L/100km. The two are reciprocals (L/100km = 100 / km/L), so a bigger km/L and a smaller L/100km both mean a thriftier car. Miles per gallon is the third common expression and follows the same direction as km/L.

The inputs are the distance you really drove and the fuel you really burned over that distance. For the pump quantity to equal the fuel burned, you need the full-tank method described next. If you think in gallons and miles, convert first (1 US gallon is 3.785 L and 1 mile is 1.609 km), or use thefuel economy unit converter.

Measuring real economy: the full-tank method

The trip computer is convenient, but its figure depends on the model and its software calibration. The most trustworthy number comes from measuring the fuel yourself.

1. Fill the tank until the pump clicks off, then reset the trip meter or write down the odometer reading.
2. Drive as you normally would. You do not have to run the tank down to the warning light, but longer is better: aim for at least 300 km (about 190 miles), because short distances magnify small errors.
3. Fill up again until the pump clicks off. Record the liters added and the distance travelled between the two fills.
4. Divide distance by liters. Ignore the quantity from your first fill: the second fill replaces what was burned since the first.

Using the same station, the same pump and the same stopping rule (no topping up after the click) cancels most of the variation in where the pump shuts off. Averaging three or four measurements is far more reliable than any single one. If all you know is the money spent, you can estimate liters as amount paid divided by price per liter, but a receipt that shows the volume to two decimals is more accurate.

Units: km/L, L/100km and mpg

Japan and Korea quote km/L, much of Europe and China quote L/100km, and the United States and the United Kingdom quote miles per gallon, but their gallons are not the same size. A US gallon is about 3.785 L and an imperial gallon is 4.546 L, so the same car has a UK mpg figure about 20% larger than its US mpg figure. A review that says "mpg" without a country can leave you misjudging a car by a fifth. The conversions used by the calculator are below, and the table is computed when the page is built.

• L/100km = 100 / (km/L)
• US mpg = km/L x 3.785412 / 1.609344, roughly km/L x 2.3521
• UK mpg = km/L x 4.54609 / 1.609344, roughly km/L x 2.8248

Worked examples

Example 1: the basic case. 450 km on 30 L is 15 km/L, which is 6.67 L/100km, 35.3 US mpg or 42.4 UK mpg. At an assumed price of 1.50 per liter the cost is 1.50 / 15 = 0.10 per km, and the fill-up itself cost 30 x 1.50 = 45.00.

Example 2: comparing with the official figure. A car rated at 13.0 km/L measures 11.2 km/L in practice. (11.2 - 13.0) / 13.0 is about -13.8%, so you are 14% below the rating. At 1.50 per liter that is about 0.115 per km on the official figure and 0.134 per km as measured, a gap of about 18.50 per 1,000 km.

Example 3: scaling to a year. A driver covering 12,000 km a year uses 1,000 L in a 12 km/L car and 1,200 L in a 10 km/L car. At an assumed 1.50 per liter that is 1,500 versus 1,800 a year: a 2 km/L gap is worth about 300 a year. Always multiply an efficiency difference by your own yearly distance before deciding whether it matters.

Cost per km at assumed fuel prices

Fuel prices move constantly, so the table uses three assumed prices (1.00, 1.50 and 2.00 per liter in your own currency) and is not a quote of any market. Each cell shows the cost per km, and below it the cost of 12,000 km, rounded to the nearest 10. Substitute your own local price in the calculator above for a real answer.

Why the official rating and your result differ

Official figures are produced on a laboratory dynamometer with a prescribed speed profile and temperature, and the prescribed procedure differs between regions. Real roads add traffic lights, hills, winter and summer climate control, payload and the driver's habits, none of which are fully captured by a standard cycle. For this reason real-world economy is usually somewhat below the label, although steady highway driving in good conditions can occasionally beat it. A shortfall of 10% or more is therefore not proof that a manufacturer overstated the number; the conditions simply are not the same. Check the regulator's own site for how ratings are defined and verified in your market.

What affects fuel efficiency

• Driving style. Hard acceleration and late braking waste energy. Smooth inputs and a generous gap to the car ahead are the cheapest improvement available.
• Speed. Aerodynamic drag rises with the square of speed, so economy falls noticeably at very high highway speeds.
• Tire pressure. Under-inflated tires increase rolling resistance. The recommended pressure is usually on a placard in the driver's door frame.
• Accessories and load. Air conditioning, extra weight and roof boxes all raise consumption. Remove carriers you are not using.
• Short trips and cold starts. An engine that never fully warms up burns more fuel per kilometer, which is a large part of the winter penalty.
• Maintenance. Dirty air filters, worn spark plugs and old oil can each cost a little.

Common mistakes when measuring

• Including the first fill. Only the fuel added at the end of the interval counts.
• Estimating liters from money. A different station means a different price and a different volume. Read the liters from the receipt.
• Topping up inconsistently. Squeezing in an extra liter on one fill and not on the next adds a one to two liter error that can swing a short interval badly.
• Judging from one short interval. Under 100 km, a tiny error in the fuel quantity changes the result a lot.
• Mixing odometers. Dashboard distance can differ slightly from navigation distance because of tire size and wear. Pick one and stay with it.

When not to use this calculator

An electric vehicle's consumption cannot be found here, because there is no liquid fuel: divide distance by the kilowatt-hours drawn from the grid instead, and for running-cost questions see theelectricity cost calculator. A plug-in hybrid burns fuel and uses electricity, so liters alone flatter it. Regulators publish an MPGe figure that treats 33.7 kWh as equivalent to one US gallon of gasoline; that compares energy content, not cost. LPG vehicles usually show lower km/L than gasoline cars but run on cheaper fuel, so compare them by cost per kilometer rather than by km/L.

Putting the result to use

• Collect three or four measurements and note winter against summer results separately; a sudden fall of more than about 10% from your usual figure can point at tire pressure, a dragging brake or an engine problem.
• When choosing between two cars, compute yearly fuel cost as distance times price divided by economy, and compare that with the purchase price difference.
• Overseas data in other units can be aligned with the table above or the fuel economy unit converter.
• For general distance and volume conversions, the unit converter is handy.

Frequently Asked Questions