40 MPG to L/100 km - Fuel Economy Conversion

One number, two gallons, three fuel economy systems

A car rated at 30 MPG in the US is actually 36 MPG in the UK - same car, same fuel, different gallon. And in Europe that same car reads 7.84 L/100 km. This converter handles all three systems: enter a value in any direction, pick US or Imperial gallon, and get instant results in every format. It also shows km/L, cost per 100 km, and classifies your fuel economy from "very efficient" to "very high."

How to convert - step by step

Conversion direction - choose MPG to L/100 km or the reverse. MPG to L/100 km is for comparing American specs to European ones.
Gallon type - US gallon (3.785 L) for US/Canadian figures, UK Imperial gallon (4.546 L) for British specs. The 20% difference is the most common source of confusion.
Value - enter the fuel economy number. EPA ratings from fueleconomy.gov use US gallons. UK V5 documents use Imperial gallons.
Read the results - the converter shows L/100 km, US MPG, UK MPG, km/L, cost estimates, and a reference table with your value highlighted.

Key numbers at a glance

235.215

US conversion constant

Divide by MPG to get L/100 km

282.481

UK conversion constant

UK gallon is 20% larger than US

3.785 L

US gallon

Used in USA, Canada, Latin America

4.546 L

UK Imperial gallon

Used in UK, older Commonwealth specs

Did you know?

MPG and L/100 km are inversely proportional. Improving from 10 to 20 MPG saves more fuel than improving from 30 to 40 MPG. Going from 10 to 11 MPG saves 2.14 L/100 km. Going from 30 to 31 MPG saves only 0.25 L/100 km.

The same car gets "better" MPG in the UK. A US-rated 30 MPG car reads 36 MPG on the UK Imperial scale because the UK gallon is 20% larger. This often confuses buyers comparing transatlantic reviews.

EPA vs WLTP ratings rarely match. EPA test cycles use more aggressive acceleration and higher speeds than WLTP. A car rated 7.0 L/100 km WLTP might test at 7.8 L/100 km on the EPA cycle - roughly 10-15% worse.

Japan uses km/L. A Japanese-spec car rated at 20 km/L equals 5.0 L/100 km or 47 MPG (US). Japanese test cycles tend to be more optimistic than EPA or WLTP.

The "MPG illusion" is real. Research by Duke University showed that people consistently underestimate the fuel savings of replacing a gas-guzzler. Swapping a 14 MPG truck for a 24 MPG sedan saves more fuel than swapping a 30 MPG car for a 50 MPG hybrid - over the same distance.

MPG to L/100 km reference table

MPG (US)	MPG (UK)	L/100 km	Typical vehicle
15	18.0	15.7	Full-size pickup, V8 muscle car
20	24.0	11.8	Large SUV, midsize truck
25	30.0	9.4	Midsize sedan, compact SUV
30	36.0	7.8	Compact sedan, hatchback
35	42.0	6.7	Efficient compact, diesel
40	48.0	5.9	Mild hybrid, small diesel
50	60.0	4.7	Full hybrid (Prius, Ioniq)
60	72.1	3.9	PHEV (electric mode blend)

Practical examples

Example 1: Ford F-150 rated at 22 MPG (US) combined.
Result: 10.69 L/100 km, 26.4 MPG (UK). Classification: above average (SUV/large sedan).

Example 2: European car at 5.2 L/100 km, convert to US MPG.
Result: 45.2 MPG (US), 54.3 MPG (UK). Classification: efficient (compact/diesel).

Example 3: UK car review claims 55 MPG (UK Imperial).
Result: 5.14 L/100 km, 45.8 MPG (US). The UK figure looks impressive but the US equivalent is 17% lower.

Example 4: Toyota Prius at 57 MPG (US) EPA combined.
Result: 4.13 L/100 km, 68.4 MPG (UK). Classification: very efficient (hybrid/PHEV).

Example 5: Dodge Ram 1500 HEMI at 15 MPG (US) city.
Result: 15.68 L/100 km, 18.0 MPG (UK). Cost: $15.68 per 100 km at $1.00/L.

FAQ - Frequently asked questions

Why are US and UK MPG numbers different for the same car?

Because the gallons are different sizes. A US gallon is 3.785 liters, while a UK Imperial gallon is 4.546 liters - about 20% larger. Since MPG measures distance per gallon, the same car covers more distance on the bigger UK gallon, producing a higher MPG number. A car rated 30 MPG in the US reads about 36 MPG in the UK.

How do I convert MPG to L/100 km manually?

Divide the conversion constant by the MPG value. For US gallons: 235.215 / MPG = L/100 km. For UK gallons: 282.481 / MPG = L/100 km. Example: 30 MPG (US) = 235.215 / 30 = 7.84 L/100 km. The reverse: 235.215 / L per 100 km = MPG.

Which system is more intuitive - MPG or L/100 km?

L/100 km is more useful for comparing costs because it scales linearly with fuel consumption. With MPG, the relationship is hyperbolic - going from 10 to 20 MPG saves far more fuel than going from 40 to 50 MPG. This "MPG illusion" (documented by Duke University researchers) causes people to underestimate savings from replacing inefficient vehicles.

Why does my car get worse MPG than the EPA rating?

EPA ratings are lab-tested under controlled conditions. Real-world factors that reduce fuel economy: aggressive acceleration (-10 to -33%), highway speeds above 50 mph (-7% per 5 mph), cold weather (-15 to -24%), roof cargo (-2 to -8%), and underinflated tires (-3%). Most drivers achieve 5-15% worse than the EPA combined rating.

How does Canada measure fuel economy?

Canada uses L/100 km as the official metric (since 1977), but many Canadians still think in MPG due to US media influence. Canadian fuel economy ratings come from Natural Resources Canada (NRCan) and use the same test cycles as the US EPA. When Canadians say "MPG," they typically mean US gallons, not Imperial.

What about km/L used in Japan and parts of Asia?

Km/L (kilometers per liter) is the inverse of L/100 km. To convert: 100 / (km/L) = L/100 km. Example: 20 km/L = 100/20 = 5.0 L/100 km = 47.0 MPG (US). Japanese JC08/WLTC test cycles tend to produce more optimistic numbers than EPA, so real-world consumption is usually 15-25% higher than the rated km/L figure.

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40 MPG - how many liters per 100 km?

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