Fuel Economy Converter

Why MPG and L/100km run in opposite directions

Miles per gallon and liters per 100 kilometers measure the same thing from opposite ends. MPG counts how far you get on a fixed amount of fuel. L/100km counts how much fuel it takes to cover a fixed distance. Raise one and the other falls, always, and not by a straight line.

That inversion is why the two scales feel unrelated. On the MPG scale a bigger number is better. On the L/100km scale a smaller number is better. A car that improves from 30 to 35 mpg has a bigger number, while the same car moves from 7.84 to 6.72 L/100km, a smaller number. Both describe one improvement.

The relationship is reciprocal rather than linear: L/100km = 235.214583 / US mpg. Doubling mpg halves the liters. Adding a fixed 5 mpg does not remove a fixed amount of fuel, which is the single most common mistake in fuel economy comparisons.

The conversion constants, and where 235.21 comes from

The constant is not magic, it is two unit conversions multiplied together.

One US gallon is 3.785411784 liters, a figure the NIST unit conversion tables define exactly. One mile is 1.609344 kilometers. To convert mpg into liters per 100 km, ask how many liters a car burns over 100 km when it covers 100/1.609344 = 62.1371 miles at mpg miles per gallon:

• Fuel used = 62.1371 / mpg gallons
• In liters = 62.1371 x 3.785411784 / mpg = 235.214583 / mpg

So the conversion is one division by the mpg figure, and the way back is the same constant divided by the L/100km figure:

• L/100km = 235.214583 / US mpg
• US mpg = 235.214583 / L/100km
• km/L = 100 / L/100km
• UK mpg = 282.480936 / L/100km

The UK constant is larger because an imperial gallon holds more fuel, not because British cars are more efficient. More on that below.

The EPA fuel economy label on a new car in the US reports mpg, and the agency's test procedures set how city and highway figures are measured. A European or Canadian spec sheet reports L/100km, and a UK one reports imperial mpg, so a shopper comparing imports is often comparing three different scales.

Worked examples: 20, 30 and 40 mpg

Three cars, one constant:

• 20 mpg: 235.214583 / 20 = 11.76 L/100km, and 100 / 11.76 = 8.50 km/L
• 30 mpg: 235.214583 / 30 = 7.84 L/100km, and 100 / 7.84 = 12.75 km/L
• 40 mpg: 235.214583 / 40 = 5.88 L/100km, and 100 / 5.88 = 17.00 km/L

Note that 20 mpg is exactly double 40 mpg in fuel used, because 11.76 is exactly double 5.88. That is the reciprocal relationship doing its job.

Now the part that surprises people. Going from 20 to 25 mpg is a 5 mpg gain, and it drops fuel use from 11.76 to 9.41 L/100km, a saving of 2.35 liters per 100 km or 20.0% of the fuel. Going from 40 to 45 mpg is the same 5 mpg gain, and it drops fuel use from 5.88 to 5.23 L/100km, a saving of 0.65 liters per 100 km or 11.1%.

Same improvement on the sticker, less than half the fuel saved. The reason is that 5 mpg is a large fraction of 20 and a small fraction of 40, and the fuel number depends on the fraction, not the difference.

Annual cost makes the point concrete. At 12,000 miles a year and $3.50 a gallon, 20 mpg burns 600 gallons for $2,100. At 25 mpg it burns 480 gallons for $1,680, a saving of $420. Moving from 40 to 45 mpg burns 267 gallons instead of 300, a saving of $117.

US and imperial gallons differ by 20%

There are two gallons in the English speaking world and they are not close. A US gallon is 3.785411784 liters. An imperial gallon, still used for fuel economy in the UK, is 4.54609 liters. The imperial gallon is 4.54609 / 3.785411784 = 1.200950 times larger, about 20.1% more fuel.

That single difference explains why a British magazine quotes 50 mpg for a car an American magazine calls 42 mpg. The British figure counts a bigger gallon, so the same car appears to travel 20% further on one gallon. Convert through liters and the confusion disappears:

• 50 UK mpg = 282.480936 / 50 = 5.65 L/100km
• 5.65 L/100km = 235.214583 / 5.65 = 41.63 US mpg

So a UK 50 mpg car is a US 41.6 mpg car, not a 50 mpg car. If you are comparing a European import with a domestic model, get both onto L/100km first, because that unit does not care which gallon you grew up with.

Canada sits in the middle of this mess historically: it sold fuel by the imperial gallon until the 1970s, then switched to liters, and its official fuel economy numbers are now L/100km. A Canadian used car listing in L/100km and a US listing in mpg can be compared directly with the constant above.

Fuel used over a year, not just per gallon

Liters per 100 km is the unit that makes annual fuel easy, because distance cancels out. Gallons per year is miles divided by mpg. Liters per year is miles converted to km, multiplied by L/100km and divided by 100.

At 12,000 miles a year and $3.50 a gallon:

The spread between 15 mpg and 30 mpg is $1,400 a year, which over five years is $7,000, more than the price difference between many used models. The spread between 40 and 50 mpg is $210 a year, which is real but rarely enough to justify a more expensive car on fuel alone.

Two adjustments keep the estimate honest. Real world mpg usually lands a few percent under the label, because the EPA test cycle cannot reproduce every driver's commute, and short trips in cold weather are worse than the test by a wide margin. And a gallon of regular has moved by more than a dollar within single years, so run the arithmetic at two prices before you decide a car is cheaper to own.

Why the same 5 mpg gain is worth less at high mpg

Insurance against the reciprocal trap: think in gallons per 100 miles, not miles per gallon. Gallons per 100 miles is 100 / mpg, and it behaves linearly, so a saving there is a saving in your wallet.

• 20 mpg = 5.00 gallons per 100 miles
• 25 mpg = 4.00 gallons per 100 miles
• 40 mpg = 2.50 gallons per 100 miles
• 45 mpg = 2.22 gallons per 100 miles

The 20 to 25 jump removes 1.00 gallon per 100 miles. The 40 to 45 jump removes 0.28 gallons. Both are 5 mpg gains on the sticker and they are not the same purchase.

The same logic applies to trading in a truck. Replacing an 18 mpg truck with a 24 mpg truck saves 6 mpg and cuts fuel use by 25%. Replacing a 36 mpg sedan with a 42 mpg sedan saves the same 6 mpg and cuts fuel use by 14%. If the goal is fuel, the low mpg vehicle is where the money is.

This is also why hybrid and electric comparisons look odd in mpg. MPGe is a gasoline equivalent based on energy content, not a real gallon burned, so a 120 MPGe electric car is not 120 miles on a gallon of gas. Compare cost per mile instead, which is what the EPA label puts in its annual fuel cost box for exactly this reason.

Reading a window sticker and a rental spec

Three reading habits cover most real situations.

On a US window sticker, the big numbers are city, highway and combined mpg. Combined is weighted 55% city and 45% highway, so a car with a large gap between the two will not match the combined figure in mostly highway driving. The sticker also lists gallons per 100 miles, which is the linear number worth comparing between cars.

On a rental car in Europe, the spec is L/100km and the fuel is sold by the liter. To sanity check the rental's economy, note the liters at fill up and the kilometers driven, then divide liters by km and multiply by 100. A car that takes 38 liters over 500 km is at 7.6 L/100km, which is 30.9 US mpg.

On a used listing in the UK, mpg means imperial mpg unless stated otherwise. Multiply by 0.8327 to get US mpg, since 1 / 1.20095 = 0.8327. A 55 imperial mpg car is 45.8 US mpg.

When a number looks impossibly good, check the unit before the engine. 4.5 L/100km is 52 US mpg, and 4.5 mpg is a bus.

Quick mental conversions that hold up

Two mental shortcuts cover the range most people drive.

MPG to L/100km: divide 235 by the mpg. 235 / 30 = 7.8, close enough to 7.84. 235 / 50 = 4.7. 235 / 15 = 15.7.

L/100km to MPG: divide 235 by the liters. 235 / 8 = 29.4 mpg. 235 / 5 = 47 mpg. 235 / 12 = 19.6 mpg.

For imperial mpg, divide 282 instead: 282 / 30 = 9.4 L/100km, which is the UK reading of the same 30 imperial mpg car.

Anchor points worth memorizing: 10 L/100km is 23.5 US mpg, 8 L/100km is 29.4, 6 L/100km is 39.2, and 5 L/100km is 47.0. In the other direction, 20 US mpg is 11.8 L/100km, 30 is 7.8, and 40 is 5.9. With those six numbers you can place any car on either scale within a few percent.

Conversion constants are exact, but fuel economy figures from manufacturers, the EPA and European regulators are measured on different test cycles and do not match one another exactly. Treat cross-market comparisons as close estimates.