You land abroad, pick up a rental car, and the spec sheet says 6.5 L/100 km. Is that good? Bad? And how much will the drive to your next stop actually cost? Here is the fuel economy math that answers all three questions in under a minute.
Two ways of measuring the same thing
Fuel efficiency is expressed in two opposite families of units, and that is where most of the confusion starts.
In the United States (and in the UK), cars are rated in miles per gallon (mpg): the distance you can drive on one unit of fuel. Higher is better. A 40 mpg car goes further on a tank than a 25 mpg car.
In most of Europe, Canada and many other countries, cars are rated in liters per 100 kilometers (L/100 km): the amount of fuel you burn to cover a fixed distance. Here, lower is better. A car at 5 L/100 km is thriftier than one at 9 L/100 km.
For reference, the average US car, pickup and SUV reached 25.4 mpg in 2020, which is about 9.3 L/100 km.
The one number you need to convert: 235
Because one unit is “distance per fuel” and the other is “fuel per distance”, you do not multiply to convert, you divide. The shortcut is a single constant:
L/100 km = 235 ÷ mpg (US), and mpg (US) = 235 ÷ L/100 km
In plain words: take 235 and divide it by the number you have, and you get the number in the other unit. It works in both directions.
There is a catch if you read British car reviews. A UK gallon (imperial gallon) holds 4.546 liters, while a US gallon holds only 3.785 liters. So “50 mpg” in a British magazine is not the same as “50 mpg” on an American window sticker. For imperial mpg, swap 235 for 282.
A real example: a 900 km road trip in a rental car
Say you usually drive a 25 mpg car at home, and your rental abroad is rated at 6.5 L/100 km.
- Is it efficient? 235 ÷ 6.5 = about 36 mpg. That is noticeably better than your 25 mpg car at home.
- How much fuel for the trip? 900 km is 9 blocks of 100 km. 9 × 6.5 = 58.5 liters.
- How much will it cost? Multiply by the price per liter shown at the pump. If fuel costs 1.90 per liter in the local currency, 58.5 × 1.90 = about 111.
That is the official figure. Now for the part most people skip.
Why you should pad the estimate
Official consumption figures come from standardized lab tests, and real-world driving is rarely that gentle. In Europe, the gap between official figures and real consumption grew from about 10% in 2001 to about 38% in 2013. Even the same car can get different official ratings depending on who tests it: the Lexus IS 250 was rated 8.9 L/100 km in the EU but 9.8 L/100 km in the United States.
So for budgeting, treat the official number as a best case. With a 38% cushion, the trip above becomes 58.5 × 1.38 = about 81 liters, or roughly 153 at 1.90 per liter. Your real bill will most likely land somewhere between the two numbers.
The mpg trap when comparing two cars
If you are choosing between rental categories, beware of what is called the “mpg illusion”. Because mpg is an inverse measure, equal gaps in mpg do not mean equal fuel savings.
Going from 10 to 20 mpg is a 100% improvement: over 100 miles you go from burning 10 gallons to burning 5, saving 5 gallons. Going from 50 to 60 mpg is only a 20% improvement: over 100 miles you go from 2 gallons to about 1.67, saving roughly a third of a gallon. Same 10 mpg difference, completely different impact on your wallet.
This is exactly why fuel-per-distance units like L/100 km are easier to compare, and why the US EPA now also shows a “gallons per 100 miles” figure on its ratings. When you compare two cars, convert both to L/100 km first: the gap you see is the fuel you actually save.
Quick recap before you hit the road
- mpg: higher is better. L/100 km: lower is better.
- Convert with 235 (US gallons) or 282 (imperial gallons), dividing in both directions.
- Trip fuel = distance in km ÷ 100 × L/100 km rating.
- Add a cushion of up to about 38% to the official figure for a realistic budget.
