MPG vs L/100 km: How to Compare Fuel Economy Fairly
A car that does "40 mpg" and a car that uses "6 liters per 100 km" sound like they belong to different worlds, but both figures answer the same question: how much fuel does it take to cover a given distance? One states the distance you get from a fixed amount of fuel, the other states the fuel you burn over a fixed distance.
This article shows how the two measures relate, why a US gallon and a UK gallon are not the same size, and why the fuel-per-distance version is often the better one for comparing vehicles. All the conversions below use exact unit definitions, so you can check them yourself.
Two ways to say the same thing
Miles per gallon (mpg) divides distance by fuel, so a bigger number means a more frugal car. Liters per 100 kilometers (L/100 km) divides fuel by distance, so a smaller number means a more frugal car. Kilometers per liter (km/L) works like mpg and is common in some countries.
Because one is the inverse of the other, the relationship is a reciprocal, not a straight multiplication. A car that is twice as thrifty has twice the mpg but half the L/100 km. That reversal catches people out when they read a table of figures in unfamiliar units.
Converting between the metric pair is simple: km/L = 100 ÷ (L/100 km). A car that uses 6.25 L/100 km therefore covers 100 ÷ 6.25 = 16 km per liter. Going from there to US mpg means multiplying by 2.352, which gives about 37.6 mpg.
US gallons and imperial gallons
The word "gallon" hides a real difference. A US gallon is exactly 3.785411784 liters. An imperial (UK) gallon is exactly 4.54609 liters, about 20% larger. A mile is 1.609344 km in both systems. Combining these gives two conversion constants:
L/100 km = 235.215 ÷ mpg (US gallons)235.215 = 100 × 3.785411784 ÷ 1.609344
L/100 km = 282.481 ÷ mpg (imperial gallons)282.481 = 100 × 4.54609 ÷ 1.609344
Worked example: a car rated at 30 US mpg uses 235.215 ÷ 30 = 7.84 L/100 km. Expressed in imperial gallons, that same 7.84 L/100 km is 282.481 ÷ 7.84 = 36.03 imperial mpg. Nothing about the car changed; only the size of the gallon did. When a UK ad quotes 48 mpg and a US sticker shows 40 mpg, they may describe near-identical efficiency.
| US mpg | Imperial mpg | L/100 km |
|---|---|---|
| 15 | 18.0 | 15.68 |
| 20 | 24.0 | 11.76 |
| 25 | 30.0 | 9.41 |
| 30 | 36.0 | 7.84 |
| 40 | 48.0 | 5.88 |
| 50 | 60.1 | 4.70 |
Always check which gallon a figure uses. Quoting an imperial mpg value as if it were US mpg overstates a car's efficiency by roughly 20%.
The same two-gallon problem affects pints and cups too; the article on US vs imperial gallons, pints, cups and spoons compares those units side by side.
Why mpg can mislead
Because mpg is fuel economy turned upside down, equal-looking gains are not equal savings. Think about 100 miles of driving:
- At 10 mpg you burn 10 gallons. At 20 mpg you burn 5. A 10 mpg gain saves 5 gallons.
- At 40 mpg you burn 2.5 gallons. At 50 mpg you burn 2. The same 10 mpg gain saves only 0.5 gallons.
Improving a thirsty vehicle by a few mpg often saves far more fuel than improving an already efficient one by a larger amount. Fuel-per-distance figures (L/100 km, or gallons per 100 miles) make this visible, because savings are simply the difference between two numbers.
Averaging has the same trap. Suppose you drive 100 miles at 20 mpg and another 100 miles at 40 mpg. The simple average of the two is 30 mpg, but you burn 5 + 2.5 = 7.5 gallons for 200 miles, so the true figure is 200 ÷ 7.5 = 26.7 mpg. Average the fuel used per distance, not the mpg values themselves.
Measuring your own fuel economy
The dashboard display is an estimate. A fill-up check is more reliable: fill the tank until the pump clicks off, reset the trip meter, drive normally, then refill the same way and note the fuel added. Divide the distance by the fuel to get mpg or km/L, or divide fuel by distance and multiply by 100 for L/100 km.
The MPG calculator does this from two odometer readings, the fuel consumption calculator works from distance and fuel in either unit system, and the fuel consumption converter switches between mpg, km/L and L/100 km in one step.
Measure over several tanks if you can. A single fill-up can be thrown off by how full the tank really was, by cold weather, or by a run of heavy traffic, while an average over a few tanks is steadier.
- Gas Mileage (MPG) Calculator: Enter your odometer readings and the amount of fuel you put in the tank to see your fuel economy in miles per gallon or liters per 100 km. Add a fuel price and the calculator also shows what each mile costs.
- Fuel Consumption Converter: Choose the unit you have, enter the figure and pick the unit you want. Twelve fuel economy and fuel consumption units are covered, including US and UK gallons.
- Fuel Consumption Calculator: Enter the distance you travelled and the fuel you burned. The calculator returns your fuel economy in litres per 100 km and kilometres per litre, or in miles per gallon, and your fuel cost per mile or kilometre if you add a price.
This article is general information, not professional advice. See our Disclaimer and Editorial Policy.
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