MPG vs L/100km: The Inverse Relationship That Fools Everyone

Fuel economy is the most commonly mishandled conversion on the road, because the two standard measures run in opposite directions. Miles per gallon is distance per fuel. Litres per 100 km is fuel per distance. They are reciprocals, and reciprocals do not behave the way intuition expects.

Higher is better, except when lower is better

• MPG: higher is better. 40 MPG beats 20 MPG.
• L/100km: lower is better. 5 L/100km beats 10 L/100km.

Because they are inverses, you cannot convert by multiplying. The conversion is:

L/100km = 235.215 ÷ MPG (US)
L/100km = 282.481 ÷ MPG (UK)

Note there are two constants, because the UK gallon is about 20% larger than the US gallon. A car rated 40 MPG in Britain is about 33 MPG in America, with no change to the car.

The MPG illusion

This is the important part, and it is genuinely counter-intuitive.

Consider driving 10,000 miles a year and two possible upgrades:

• 10 MPG → 20 MPG: 1000 gallons → 500 gallons. Saves 500 gallons.
• 30 MPG → 60 MPG: 333 gallons → 167 gallons. Saves 166 gallons.

Both upgrades double efficiency. The first saves three times as much fuel.

Even more striking: going from 12 to 14 MPG (a 2 MPG gain) saves more fuel over 10,000 miles than going from 30 to 40 MPG (a 10 MPG gain). Replacing an old truck matters more than perfecting an already-efficient car.

L/100km does not have this problem. It is linear in what you actually care about, which is fuel burned. Halving L/100km always halves consumption. This is precisely why most of the world and the EU labelling regime use it.

You cannot average MPG

A related and very common error.

Drive 100 miles at 50 MPG, then 100 miles at 10 MPG. The average is not 30 MPG.

Work in fuel: the first leg uses 2 gallons, the second uses 10. That is 200 miles on 12 gallons, or 16.7 MPG.

The correct method is the harmonic mean, not the arithmetic mean. Or simply convert to L/100km, average those (they can be averaged over equal distances), and convert back.

Other measures you will meet

• km/L — common in Japan, India and parts of South America. Higher is better, like MPG.
• MPGe — for electric vehicles, based on the energy content of a gallon of petrol (33.7 kWh). Useful for comparing across fuel types, but it says nothing about your electricity cost.
• Wh/mile or kWh/100km — the honest EV measure, analogous to L/100km.

Why real economy differs from the sticker

Official figures come from standardised test cycles, and real driving is not a test cycle:

• Speed. Aerodynamic drag rises with the square of speed, and power with the cube. 80 mph can use 20–30% more fuel than 65 mph.
• Cold starts. A cold engine runs rich. Short trips are disproportionately thirsty.
• Tyre pressure. Under-inflation adds rolling resistance.
• Air conditioning. Several percent, more in heavy traffic.
• Winter fuel blends carry slightly less energy per litre.

Practical takeaways

1. To compare cars honestly, convert both to L/100km, or to fuel used over a fixed annual distance.
2. Upgrading the least efficient vehicle you own saves the most fuel.
3. Never average MPG figures directly.
4. Check whether a quoted MPG is US or UK before comparing.

Our fuel consumption converter handles the inverse relationship correctly and keeps US and UK gallons separate.

Measuring your own economy properly

Trip computers are convenient and frequently optimistic, sometimes by 5–10%. The tank-to-tank method is more work and considerably more trustworthy.

1. Fill the tank until the pump clicks off. Do not top up beyond the first click — consistency matters more than fullness.
2. Reset the trip odometer, or note the total mileage.
3. Drive normally for at least one full tank. Short samples exaggerate the effect of any single unusual journey.
4. Refill at the same pump if you can, again to the first click.
5. Divide distance travelled by fuel added.

Averaging three or four consecutive tanks smooths out pump variation and gives a figure you can actually rely on for comparison.

Note that this measures distance per fuel added, which equals distance per fuel used only because you returned the tank to the same level. That is the whole reason the first-click rule matters.

What actually improves economy

Ranked roughly by effect for typical driving:

• Reduce motorway speed. Drag rises with the square of speed. Dropping from 80 to 70 mph often gains 10–15%.
• Smooth out acceleration and braking. Every braking event discards energy you paid for.
• Combine short trips. A cold engine can use twice the fuel of a warm one for the first few miles.
• Keep tyres at the door-placard pressure. Worth a few percent, and free.
• Remove roof boxes and bars when not in use. At motorway speed a roof box can cost 10–25%.
• Shed unnecessary weight. Real but modest, and it matters most in stop-start driving rather than at steady speed.