Who needs this?
Fuel numbers get converted whenever a car, a driver and a gas pump come from different places. A few people do it constantly:
- Road trippersSplitting gas money for a weekend drive and want a fair number before anyone pays anyone back.
- Travelers renting abroadThe rental in Portugal is rated 5.8 L/100 km and fuel is sold by the liter in euros.
- Car shoppersComparing a US window sticker with a UK review or a Canadian spec sheet.
- Fleet and work trucksDeciding which vehicles to replace first to cut the fuel bill the most.
- CommutersChecking whether a longer drive to a cheaper station actually saves anything.
- EV-curious driversMaking sense of MPGe and kWh per 100 miles next to plain mpg.
Two ways to measure the same thing
Think of two ways to describe a runner: laps per hour, or minutes per lap. Same runner, same speed, but one number goes up when they get faster and the other goes down. Fuel economy works the same way.
- Miles per gallon (mpg) and kilometers per liter (km/L) are distance per fuel. Higher is better. The US uses mpg; km/L is common in Japan, India and much of Latin America.
- Liters per 100 km (L/100 km) is fuel per distance. Lower is better. Europe, Canada, Australia and China rate cars this way.
- UK mpg uses the imperial gallon, 4.54609 L, which is about 20% bigger than the US gallon (3.785411784 L). The same car scores a higher mpg in Britain without using any less fuel.
Because one is upside down from the other, the conversion is a division. The 235.215 is just the unit change folded into one number: 100 × 3.785411784 ÷ 1.609344. Swap in the UK gallon and it becomes 282.481.
The upside-down shape has a consequence most people never notice. Each extra mpg saves less fuel than the one before it, as the curve below shows.
Converting and pricing a drive, step by step
mpg to L/100 km (and back)
Divide 235.215 by the number you have. The same division works in both directions, because the conversion is its own reverse.
- Divide 235.215 by the mpg
235.215 ÷ 30 = 7.84 L/100 km - Going back, divide 235.215 by the L/100 km
235.215 ÷ 8 = 29.4 mpg
For km/L, divide 100 by the L/100 km figure (100 ÷ 8 = 12.5 km/L), or multiply US mpg by 0.425. For a British figure, use 282.481 in place of 235.215, or multiply UK mpg by 0.8327 to get US mpg.
What a drive costs
Find the fuel first, then multiply by the price. With mpg, fuel is distance ÷ mpg. With L/100 km, it's distance ÷ 100 × the rating. Keep the price in the same unit as the fuel: dollars per gallon with gallons, euros per liter with liters. The volume guide has the gallon and liter math if a price is in the "wrong" unit.
- Fuel used
300 mi ÷ 28 mpg = 10.71 gal - Times the price
10.71 gal × $3.50 = $37.50 - Round trip
double the distance first: $75.00
Distance: 300 miTrip: Round tripFuel economy: 28Fuel economy unit: Miles per gallon (US)Fuel price: $3.50Fuel price is: Per gallon (US)Split between: 1 people
Real situations, worked out
The car gets 32 mpg and gas is $3.29. Round trip is 480 miles: 480 ÷ 32 = 15 gallons, and 15 × $3.29 = $49.35. Split three ways, that's $16.45 each. Whether the driver chips in is between friends.
$49.35 total · $16.45 eachDistance: 240 miTrip: One wayFuel economy: 32Fuel economy unit: Miles per gallon (US)Fuel price: $3.29Fuel price is: Per gallon (US)Split between: 3 people
The drive is 650 km. Fuel used is 650 ÷ 100 × 6.5 = 42.25 liters, and 42.25 × 1.85 = €78.16. No gallons needed: L/100 km and a per-liter price fit together directly. (6.5 L/100 km is about 36.2 US mpg, if you want to compare it with the car at home.)
42.25 L · about €78A business drives two vehicles 12,000 miles a year each. Option A swaps a 15 mpg truck for a 20 mpg one. Option B swaps a 35 mpg sedan for a 50 mpg hybrid. Option B sounds bigger (+15 mpg against +5), but the truck goes from 800 to 600 gallons a year (saving 200) while the sedan goes from about 343 to 240 (saving about 103). At $3.50 a gallon, the truck saves $700 a year and the hybrid about $360.
the "small" 5 mpg upgrade saves almost twice as much1,200 miles at 25 mpg and $3.40 a gallon is 48 gallons, or $163.20 of fuel. Add 3 nights at $150 ($450), 4 days of food at $90 for the group ($360) and $60 of tolls and parking, and the trip comes to $1,033.20, or $258.30 each. Fuel is the smallest of the big lines.
$1,033.20 total · $258.30 per personTotal distance: 1,200 miFuel economy: 25Fuel economy unit: Miles per gallon (US)Fuel price: $3.40Fuel price is: Per gallon (US)Nights: 3Lodging per night: $150.00Days: 4Food per day (whole group): $90.00Tolls, parking & extras: $60.00People: 4
Doing it in your head
For mpg to L/100 km, divide 240 by the mpg. 240 splits evenly by 10, 12, 15, 20, 24, 30, 40 and 60, so most answers come out clean. It always reads about 2% high, which is far less than the gap between a car's rating and real driving.
| US mpg | Exact L/100 km | 240 ÷ mpg | Off by |
|---|---|---|---|
| 15 | 15.68 | 16.0 | 0.32 |
| 20 | 11.76 | 12.0 | 0.24 |
| 25 | 9.41 | 9.6 | 0.19 |
| 30 | 7.84 | 8.0 | 0.16 |
| 40 | 5.88 | 6.0 | 0.12 |
| 50 | 4.70 | 4.8 | 0.10 |
Compare in fuel, not in mpg. To judge savings, turn each mpg into gallons per 1,000 miles (1,000 ÷ mpg) and subtract. 20 mpg is 50 gallons; 30 mpg is 33.3. The difference is the fuel you'd actually save.
Common mistakes
Averaging mpg figures. A car that does 100 miles at 20 mpg and 100 miles at 40 mpg didn't average 30 mpg. It used 5 + 2.5 = 7.5 gallons for 200 miles, which is 26.7 mpg. Add up the fuel and the distance, then divide.
Treating mpg gains as equal. Going from 10 to 20 mpg saves 50 gallons over 1,000 miles. Going from 30 to 50 saves about 13.3. Even 20 to 30 (16.7 gallons) beats 30 to 50. Researchers call this the "mpg illusion".
Mixing up US and UK mpg. A British review quoting 55 mpg means about 45.8 US mpg (5.14 L/100 km). Comparing it straight against an American sticker flatters the British car by about 20%.
Pricing liters with a per-gallon price. In Canada fuel is sold by the liter, so $1.60 is a price per liter. Multiplying gallons by that number undercounts the cost by a factor of almost four (3.785).
A bit of history
Fuel economy rules in the US grew out of the 1973 oil crisis, and the labels have been rethought several times since.
- 1975Congress passes the Energy Policy and Conservation Act, which creates the Corporate Average Fuel Economy (CAFE) standards. The first ones applied to 1978 model-year cars.
- 2008The EPA changes how it calculates the ratings for 2008 models, adding faster driving, air conditioning and cold weather. Most cars' numbers drop.
- 2008Richard Larrick and Jack Soll publish "The MPG Illusion" in Science, showing that most people misjudge which upgrades save the most fuel. They suggest labeling cars in gallons per 100 miles.
- 2011The EPA and NHTSA announce redesigned window stickers, required from the 2013 model year. They show gallons per 100 miles next to mpg, and MPGe for electric cars.
Electric cars use the same logic. MPGe counts 33.7 kWh of electricity as one gallon of gasoline, so a car that uses 30 kWh per 100 miles rates about 112 MPGe. The kWh per 100 miles figure is the useful one for cost: at $0.16 per kWh, those 100 miles cost $4.80, against about $11.67 for a 30 mpg car on $3.50 gas. The energy and power guide explains kWh.
Reference table
| US mpg | L/100 km | km/L | UK mpg | Gallons per 1,000 mi |
|---|---|---|---|---|
| 12 | 19.6 | 5.1 | 14.4 | 83.3 |
| 15 | 15.7 | 6.4 | 18.0 | 66.7 |
| 20 | 11.8 | 8.5 | 24.0 | 50.0 |
| 25 | 9.4 | 10.6 | 30.0 | 40.0 |
| 30 | 7.8 | 12.8 | 36.0 | 33.3 |
| 35 | 6.7 | 14.9 | 42.0 | 28.6 |
| 40 | 5.9 | 17.0 | 48.0 | 25.0 |
| 50 | 4.7 | 21.3 | 60.0 | 20.0 |
| 60 | 3.9 | 25.5 | 72.1 | 16.7 |
Questions people ask
Is 7 L/100 km good?
It's about 33.6 US mpg, which is efficient for a gas car without a hybrid system. For comparison, a large pickup often lands around 12 to 14 L/100 km.
Why don't I get the mpg on the sticker?
The rating comes from standard lab tests. Real driving adds things the test averages out: highway speeds above 65 mph or so, air conditioning, cold starts, roof racks, a loaded trunk, short trips and underinflated tires. Use your own fill-up records for trip math when you have them.
How do I work out my real mpg?
Fill the tank and zero the trip meter. At the next fill-up, divide the miles driven by the gallons it took. A few tanks averaged together (total miles ÷ total gallons) beat any single reading.
Why is UK mpg higher than US mpg for the same car?
The imperial gallon is 4.54609 L and the US gallon is 3.785411784 L, so a UK gallon takes a car about 20% farther. Multiply US mpg by 1.201 to get UK mpg.
What does MPGe mean?
Miles per gallon equivalent. It treats 33.7 kWh of electricity as one gallon of gasoline so EVs can sit on the same scale. For running costs, the kWh per 100 miles figure and your electricity rate tell you more.