MPG vs L/100km: Fuel Economy Units Finally Explained

Published on June 2, 2026 · 8 min read

Walk into a car showroom in the US and the salesperson talks about miles per gallon. Walk into one in Europe and every brochure quotes litres per 100 kilometres. They both describe how thirsty a car is, but they do it from opposite directions — and that is where most people get confused. This guide unpacks both units, gives you a clean conversion formula, and explains why "double the MPG" does not mean "half the fuel".

Two ways to measure the same thing

Fuel economy is a ratio between distance travelled and fuel used. There are two ways to frame it:

• MPG (miles per gallon): distance per unit of fuel. Bigger is better.
• L/100 km (litres per 100 kilometres): fuel per unit of distance. Smaller is better.

They are mathematical inverses of each other (with a unit conversion thrown in). MPG asks "how far can I go on this much fuel?" while L/100 km asks "how much fuel does this trip cost me?". European regulators chose L/100 km because it scales linearly with cost — twice the figure means twice the fuel bill. MPG does not.

The conversion formula

Use this single formula either direction — it is symmetrical:

The constants are different because the US gallon (3.785 L) is smaller than the imperial gallon (4.546 L) used in the UK. To go the other way, divide the constant by L/100 km. We covered the gallon difference in detail in our US vs imperial gallon guide.

Quick reference table

The numbers below use US MPG, which is what most North American manufacturers publish:

• 15 MPG ≈ 15.7 L/100 km — large pickup truck or SUV
• 20 MPG ≈ 11.8 L/100 km — older sedan or small SUV
• 25 MPG ≈ 9.4 L/100 km — average modern sedan
• 30 MPG ≈ 7.8 L/100 km — efficient compact car
• 40 MPG ≈ 5.9 L/100 km — small diesel or hybrid
• 50 MPG ≈ 4.7 L/100 km — full hybrid like a Prius
• 60 MPG ≈ 3.9 L/100 km — best modern diesels

Why "doubling MPG" is misleading

The relationship between MPG and fuel saved is non-linear, and this trips up almost everyone. Consider two upgrades on a 15,000 km/year driver:

• Going from 15 → 25 MPG saves about 400 litres of fuel per year.
• Going from 40 → 50 MPG — the same 10-MPG jump — saves only about 75 litres per year.

The L/100 km figure makes this obvious: 15.7 → 9.4 is a huge drop; 5.9 → 4.7 is barely visible. If you actually care about emissions or fuel bills, replacing a thirsty old vehicle with a moderately efficient one matters far more than swapping a hybrid for a slightly better hybrid.

How to measure your real-world economy

Trip computers are convenient but often optimistic. To get an honest figure, do the "brim-to-brim" method:

1. Fill the tank completely. Note the odometer reading.
2. Drive normally for at least 500 km / 300 miles.
3. Fill the tank completely again at the same pump style if possible. Record litres or gallons added and the new odometer reading.
4. Distance ÷ fuel used = real economy. Repeat over 3–4 tanks for a reliable average.

What hurts your real fuel economy

Manufacturer figures come from controlled lab tests. The real world includes:

• Speed: Above 100 km/h aerodynamic drag dominates. Going from 110 to 130 km/h can cost 15–20% efficiency.
• Cold starts: The first 5 km after a cold start can consume fuel at twice the steady-state rate.
• Tyre pressure: Under-inflated tyres add rolling resistance. Check pressures monthly.
• Roof racks: An empty roof box can add 10–15% to highway consumption.
• Air conditioning: Worth 3–10% on hot days, but at highway speeds usually better than open windows.

EV equivalents

Electric vehicles use a different unit — kWh per 100 km in Europe, or MPGe (miles per gallon equivalent) in the US, where 33.7 kWh is treated as energy-equivalent to one gallon of petrol. A Tesla Model 3 long-range consumes about 15 kWh/100 km, equivalent to roughly 130 MPGe. Apples-to-apples comparisons get tricky because the upstream energy losses (refinery vs power plant) are very different.

Quick conversion with ConvertProf

Convert between gallons and litres in seconds with the ConvertProf volume converter, or use the scientific calculator to apply the 235.215 / MPG formula directly. For trip planning, pair this with our kilometres-to-miles guide so the distance units line up with the fuel units.

FAQ

Why is the same car rated differently in the US and UK?

Different gallon sizes (US 3.785 L vs UK 4.546 L) and different test cycles (EPA vs WLTP). A UK MPG figure will always look higher than the equivalent US MPG figure for the same car.

Is 30 MPG good?

For a midsize sedan in 2026 it is average. For a full-size truck it is excellent. For a small hybrid it is disappointing.

Do diesel and petrol use the same unit?

Yes — both are quoted in MPG or L/100 km. But diesel has about 13% more energy per litre, which is one reason diesel cars usually post better fuel-economy numbers.

Bottom line

L/100 km is the more honest unit because it scales linearly with cost and CO₂. MPG is more intuitive if you grew up with it but hides the diminishing returns of efficiency improvements at the top end. Whichever one your dashboard speaks, the conversion is a single division — and ConvertProf can do it in a single tap.