How fuel economy is measured
Fuel economy is the distance a vehicle travels per unit of fuel. The cleanest way to measure it is to fill the tank, drive, then fill it again: the miles on the trip meter divided by the gallons it took to top off is your real-world economy for that stretch of driving.
The same physical efficiency can be written several ways. Miles per gallon (used in the US) gets bigger as a car gets more efficient. The metric L/100km flips that around — it counts fuel used per fixed distance, so smaller is better. Kilometres per litre behaves like MPG, just in metric units.
MPG and L/100km point in opposite directions
Because L/100km is fuel-per-distance and MPG is distance-per-fuel, they are inverses of each other (L/100km = 235.215 / MPG with US gallons). This is why a jump from 20 to 30 MPG saves far more fuel than a jump from 40 to 50 MPG, even though both are a ten-MPG gain. Comparing in L/100km or gallons-per-100-miles makes the real savings obvious.
Getting an accurate reading
A single tankful can be skewed by how it was driven and how full each fill was.
- Fill to the same click-off point both times so the gallons figure is consistent.
- Average several tanks across mixed conditions for a number you can trust.
- Expect lower economy in winter, in heavy traffic, and with a loaded or roof-racked vehicle.
- Reset the trip meter at the first fill so the distance lines up exactly with the fuel used.
Turning economy into cost
Add a fuel price and the calculator also shows cost per mile, which converts efficiency into money. That rate is the most practical way to compare vehicles or decide whether a more efficient car will pay back its price difference over the miles you actually drive.
Formula
mpg = miles / gallonsFrequently asked questions
- How do I convert MPG to L/100km?
- L/100km = 235.215 / MPG (using US gallons). Lower L/100km means better efficiency, the opposite of MPG.