Understanding Fuel Economy: MPG, L/100km, and What Actually Costs You
Going from 15 to 20 MPG saves more fuel than going from 40 to 60. That is not a typo, and it is why MPG is the wrong way round.
By Rohit Sharma, Founder, SEOShouts
Fuel economy is measured two ways. Miles per gallon asks how far you travel per unit of fuel. Litres per 100 kilometres asks how much fuel you burn per unit of distance. They describe the same thing, but one of them lies to your intuition.
The MPG illusion
MPG is a ratio with distance on top, so improvements at the low end are worth far more than improvements at the high end. Compare two upgrades over 10,000 miles.
| Change | Fuel before | Fuel after | Saved |
|---|---|---|---|
| 15 → 20 MPG | 667 gal | 500 gal | 167 gal |
| 40 → 60 MPG | 250 gal | 167 gal | 83 gal |
A 5 MPG improvement on a thirsty vehicle saves twice as much fuel as a 20 MPG improvement on an efficient one. Replacing the worst vehicle in a household is almost always the better decision, even though the headline MPG gain looks smaller.
L/100km does not have this problem. Fuel is on top, so the number moves in proportion to what you actually spend. This is why most of the world, and most engineers, prefer it.
Converting between them
L/100km = 282.48 ÷ MPG
For US gallons, which are about 17 per cent smaller, use 235.21 ÷ MPG instead.
The conversion is its own inverse, so the same constant converts back. 40 imperial MPG is 282.48 ÷ 40 = 7.06 L/100km, and 282.48 ÷ 7.06 returns 40.
What actually moves fuel consumption
Aerodynamic drag rises with the square of speed, and the power needed to overcome it rises with the cube. That single fact dominates motorway economy.
- Speed. Going 80 mph instead of 70 typically costs 15 to 20 per cent more fuel over the same distance.
- Tyre pressure. Under-inflation by 10 psi costs around 3 per cent, and it wears tyres faster.
- Roof boxes and bars. An empty roof rack can cost 10 per cent on the motorway. Remove it when not in use.
- Weight. Roughly 1 to 2 per cent per 50 kg, which matters mostly in stop-start driving.
- Cold starts. The first few miles can consume at double the normal rate, so short trips are disproportionately expensive.
- Harsh acceleration and braking. Every hard stop discards energy you paid for.
Why you never match the official figure
Official economy figures come from standardised laboratory cycles run at controlled temperature with no headwind, no passengers, no luggage and no air conditioning. They exist to let vehicles be compared on equal terms, not to predict your commute. Real-world consumption commonly runs 10 to 25 per cent worse, and more than that in cold weather or heavy traffic.
The reliable way to know your own figure is to measure it. Fill the tank completely, reset the trip meter, drive normally until you need fuel again, then fill completely once more. Divide the distance travelled by the fuel added. Repeat over a few tanks and average, since a single tank is sensitive to how precisely the pump cut off.
Frequently asked questions
Is MPG or L/100km better?
L/100km is the more honest scale because it moves in proportion to fuel actually used. MPG understates how much you gain from improving an inefficient vehicle, since it puts distance rather than fuel on top of the ratio.
How do I convert MPG to L/100km?
Divide 282.48 by the imperial MPG figure, or 235.21 by the US MPG figure. The same formula converts back the other way.
Why is my real fuel economy worse than the official rating?
Official figures come from controlled laboratory cycles without passengers, luggage, air conditioning, wind or cold starts. Real-world consumption is typically 10 to 25 per cent higher, and worse again in winter or heavy traffic.
Calculators from this guide
About the author
Rohit Sharma is the founder of SEOShouts, a search consultancy in India, and has worked in technical SEO and content strategy since 2014. He builds and maintains Calcshark.