Does going from 10 to 20 mpg save more fuel than going from 33 to 50 mpg? It saves nearly five times as much, and most drivers guess the opposite. To convert mpg to L/100km, divide 235.215 by the US mpg figure. This guide explains where that number comes from, the UK gallon trap, and why fuel-per-distance units compare cars more honestly.
- L/100km equals 235.215 divided by US mpg, and US mpg equals 235.215 divided by L/100km.
- For UK (imperial) mpg, use 282.481 instead, because the imperial gallon holds 4.54609 liters.
- 30 US mpg equals 7.84 L/100km, while 30 UK mpg equals 9.42 L/100km.
- km/L is a third unit: multiply US mpg by 0.425144, and L/100km equals 100 divided by km/L.
- Fuel used per distance compares cars better than mpg, because mpg gains shrink as mpg rises.
What Is the Formula to Convert MPG to L/100km?
Divide 235.215 by the US miles per gallon figure to get liters per 100 kilometers. The same constant works in reverse: divide 235.215 by L/100km to get US mpg.
The two units are reciprocals, so one fixed number links them. MPG measures distance per unit of fuel. L/100km measures fuel per unit of distance, so a lower number means a thriftier car.
Where Does 235.215 Come From?
The constant combines two exact definitions. One US gallon is 3.785411784 liters, and one mile is exactly 1.609344 kilometers. Multiply 100 by 3.785411784 and divide by 1.609344, and you get 235.2146, rounded to 235.215.
NIST publishes the same rule in its official conversion guide: divide 235.215 by the number of miles per gallon. It follows directly from the legal gallon and mile.
Two Quick Worked Examples
A sedan rated at 30 US mpg uses 235.215 / 30 = 7.84 L/100km. Going the other way, a European hatchback rated at 6.5 L/100km returns 235.215 / 6.5 = 36.19 US mpg.
The rating itself comes from standardized lab cycles. Our guide on what MPG means and how it is tested covers that side.
How Do UK MPG and US MPG Differ?
UK mpg uses the imperial gallon of 4.54609 liters, which is about 20 percent larger than the US gallon. The same car therefore shows a higher mpg number in the UK than in the US.
The imperial constant is 100 x 4.54609 / 1.609344 = 282.481. To convert UK mpg to L/100km, divide 282.481 by the UK figure. One UK mpg equals 1.20095 US mpg.
The Most Common Conversion Error
Many drivers read a British review and apply the US constant. Take a car rated at 30 UK mpg. The correct answer is 282.481 / 30 = 9.42 L/100km.
Using 235.215 instead gives 7.84 L/100km, which understates real fuel use by 1.58 L/100km, or 16.7 percent. That same 30 UK mpg car only manages 24.98 US mpg. Always check which gallon a rating uses before you convert.
Where Does km/L Fit In?
Kilometers per liter is a distance-per-fuel unit used in Japan, India, and much of Latin America. Multiply US mpg by 0.425144 to get km/L, and divide 100 by km/L to get L/100km.
At 30 US mpg, a car covers 12.75 km per liter, and 100 / 12.75 = 7.84 L/100km. Like mpg, km/L rises as efficiency improves. Only L/100km runs the other way.
The table below lists common ratings computed from the exact definitions. For any other value, the Fuel Economy Converter switches between all four units instantly.
| US mpg | L/100km | km/L | UK mpg |
|---|---|---|---|
| 20 | 11.76 | 8.50 | 24.02 |
| 25 | 9.41 | 10.63 | 30.02 |
| 30 | 7.84 | 12.75 | 36.03 |
| 35 | 6.72 | 14.88 | 42.03 |
| 40 | 5.88 | 17.01 | 48.04 |
| 50 | 4.70 | 21.26 | 60.05 |
Notice the L/100km column. Each 5 mpg step saves less fuel than the step before it.
What Is the MPG Illusion?
The MPG illusion is the mistaken belief that fuel savings grow evenly with mpg. In reality, each extra mpg saves less fuel than the one before, because fuel use is the reciprocal of mpg.
Richard Larrick and Jack Soll described this bias in the journal Science in 2008. They found that people judge fuel savings as if they rose in a straight line with mpg. The math shows a steep curve instead.
Compare two upgrades. Going from 10 to 20 mpg cuts fuel from 10 to 5 gallons per 100 miles, saving 5 gallons. Going from 33 to 50 mpg cuts fuel from 3.03 to 2.00 gallons, saving only 1.03 gallons.
The second upgrade adds 17 mpg, while the first adds only 10. Yet the first saves almost five times as much fuel. The curve below shows why the low end matters most.
Why Is L/100km Better for Comparing Cars?
L/100km measures fuel used, which is what you pay for, so differences between cars scale directly with cost. MPG differences do not, because mpg sits in the denominator.
Larrick and Soll recommended expressing efficiency as gallons per 100 miles for this reason. The US fuel economy label on new cars now pairs mpg with a fuel consumption figure per 100 miles. Canada, Europe, and Australia rate cars in L/100km.
Consumption units also make subtraction honest. A 20 mpg truck and a 25 mpg truck differ by 1.00 gallon per 100 miles. A 40 mpg hybrid and a 50 mpg hybrid differ by only 0.50 gallons, despite the bigger 10 mpg gap.
A Worked Yearly Example at 12,000 Miles
Picture a driver covering 12,000 miles a year. A 25 mpg car burns 12,000 / 25 = 480 gallons, or 1,817 liters. A 40 mpg car burns 300 gallons, or 1,136 liters.
The switch saves 180 gallons a year. At the illusion extremes, going from 10 to 20 mpg saves 600 gallons, while 33 to 50 mpg saves 123.6 gallons. To price a single trip, see our guide on estimating fuel cost for a road trip.
How Do You Avoid Mistakes When Converting?
Confirm the gallon type, keep the direction straight, and round only at the end. These three habits prevent nearly every conversion error.
First, check the source country. American figures use US gallons, while British and older Canadian figures use imperial gallons. Second, remember that a lower L/100km is better. A car at 5.5 L/100km beats one at 6.5 L/100km.
Third, carry full precision through each step. Rounding 235.215 down to 235 shifts a 30 mpg result by only 0.01 L/100km. Rounding the mile to 1.6 km, however, pushes the constant to 236.59, a 0.6 percent error.
Finally, compare consumption, not mpg, when choosing between two cars. Convert both to L/100km or gallons per 100 miles, then subtract. The difference times your yearly distance gives the real fuel saved.
The Fuel Economy Converter switches instantly between US mpg, UK mpg, km/L, and L/100km using the exact gallon and mile definitions.
FAQs About Converting Fuel Economy
How Do You Convert MPG to L/100km?
Divide 235.215 by the US mpg figure. For example, 25 US mpg equals 235.215 divided by 25, which is 9.41 L/100km. For UK mpg, divide 282.481 by the figure instead.
What Is 30 MPG in L/100km?
Thirty US mpg equals 7.84 L/100km. Thirty UK mpg equals 9.42 L/100km, because the imperial gallon is about 20 percent larger than the US gallon.
How Do You Convert L/100km Back to MPG?
Use the same constant in reverse. Divide 235.215 by the L/100km figure to get US mpg, or divide 282.481 by it to get UK mpg. A car at 6.5 L/100km returns 36.19 US mpg.
Is UK MPG the Same as US MPG?
No. UK mpg uses the 4.54609 liter imperial gallon, while US mpg uses the 3.785411784 liter US gallon. One UK mpg equals 1.20095 US mpg, so UK ratings look about 20 percent higher.
Why Is a Lower L/100km Number Better?
L/100km measures fuel burned over a fixed distance. A smaller number means the car needs less fuel to cover 100 kilometers, so it costs less to run.
What Is the MPG Illusion in Simple Terms?
It is the false belief that each extra mpg saves the same amount of fuel. Going from 10 to 20 mpg saves 5 gallons per 100 miles, while 33 to 50 mpg saves only 1.03 gallons.
Should I Compare Two Cars in MPG or L/100km?
Compare them in L/100km or gallons per 100 miles. Those units show fuel used directly, so the difference between two cars scales with your real fuel cost.
Sources
References Used in This Article
This article covers fuel economy unit conversion and the math behind comparing cars. Real-world fuel use varies with driving style, load, and weather. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated September 27, 2026.
Author
Shakeel Muzaffar is the Founder and Editor-in-Chief of MultiCalculators.com, bringing over 15 years of experience in digital publishing, product strategy, and online tool development. He leads the platform's editorial vision, ensuring every calculator meets strict standards for accuracy, usability, and real-world value. Shakeel personally oversees content quality, formula verification workflows, and the platform's commitment to publishing tools that are genuinely useful for students, professionals, and everyday users worldwide.
