MPG vs. L/100km: Why Fuel Economy Conversion Isn't a Simple Multiplication

Most unit conversions are a straightforward multiplication: multiply by a fixed factor and you’re done. Fuel economy is different, and it trips people up because the two most common units — miles per gallon (MPG) and liters per 100 kilometers (L/100km) — measure opposite things.

Higher is better vs. lower is better

MPG measures distance per unit of fuel — how far you can travel on a fixed amount of fuel. A higher MPG means better efficiency (you go further on the same amount of fuel).

L/100km measures fuel per unit of distance — how much fuel you burn over a fixed distance. A lower L/100km means better efficiency (you burn less fuel to cover the same distance).

Because one is “distance per fuel” and the other is “fuel per distance,” converting between them is an inverse (reciprocal) relationship, not a direct multiplication. You can’t multiply an MPG figure by a constant to get L/100km — you have to invert it.

The actual formula

For US gallons:

L/100km = 235.215 / MPG

And the reverse:

MPG = 235.215 / (L/100km)

The constant 235.215 comes from converting gallons to liters and miles to kilometers and folding those conversions into a single factor for the reciprocal relationship. (For imperial gallons, used in the UK, the constant is different — about 282.5 — because an imperial gallon is larger than a US gallon. See below.)

Worked example

A car rated at 30 MPG (US):

L/100km = 235.215 / 30 = 7.84 L/100km

Now suppose fuel economy improves to 40 MPG:

L/100km = 235.215 / 40 = 5.88 L/100km

Notice that a 10 MPG improvement (33% increase in MPG) only reduced L/100km by about 1.96 — and the size of the L/100km improvement for a fixed MPG gain depends on where you start. This non-linearity is the second major reason MPG and L/100km don’t convert with a simple ratio: equal steps in one unit don’t correspond to equal steps in the other.

This has a real-world implication: the fuel savings from improving a car’s MPG from 15 to 20 (an increase of 5 MPG) are much larger than the savings from improving from 35 to 40 (also an increase of 5 MPG). In L/100km terms, going from 15 to 20 MPG drops fuel use from 15.7 to 11.8 L/100km — a savings of 3.9 L/100km. Going from 35 to 40 MPG only drops it from 6.7 to 5.9 L/100km — a savings of 0.8 L/100km. The same MPG improvement saves nearly 5 times as much fuel at the low end of the scale.

US gallons vs. imperial gallons

To make things more complicated, “MPG” doesn’t mean the same thing everywhere:

• US MPG uses the US gallon (3.785 liters).
• Imperial MPG (used in the UK) uses the imperial gallon (4.546 liters) — about 20% larger.

Because the imperial gallon is bigger, the same car will show a higher “MPG” figure when measured in imperial gallons than in US gallons, even though the actual fuel consumption (and distance traveled) is identical. A car that gets 30 US MPG would show roughly 36 imperial MPG — not because it’s more efficient, but because each “gallon” represents more fuel.

This is why comparing a US car’s advertised MPG directly to a UK car’s advertised MPG can be misleading without knowing which gallon definition was used. Converting both figures to L/100km — a metric, gallon-independent unit — removes this ambiguity entirely, which is one reason L/100km is the standard for fuel economy labeling across most of the world.

Quick reference table (US gallons)

MPG (US)	L/100km
20	11.76
25	9.41
30	7.84
35	6.72
40	5.88
50	4.70

Practical takeaway

When comparing fuel economy figures from different sources — especially across countries — convert everything to L/100km first using the MPG to L/100km converter, check which gallon definition (US or imperial) the original MPG figure used, and remember that because the relationship is inverse, a “small” difference in MPG at high efficiency represents a much smaller real-world fuel savings than the same numeric difference at low efficiency.