Miles per gallon and liters per 100 kilometers measure fuel economy in opposite directions. Cost comparison becomes clearer when distance, fuel price, and consumption are treated consistently.
The direction trap
Higher MPG means more distance per gallon and is generally better. Lower L/100 km means fewer liters used for the same distance and is generally better. Do not compare the numbers without their units.
Convert to a trip cost
For a fixed trip, estimate fuel used from distance and economy, then multiply by the local fuel price. A small difference in consumption matters more over many miles or kilometers.
Use realistic driving assumptions
Official ratings may differ from real use because of speed, weather, load, tires, traffic, and driving style. Compare vehicles under the same distance and price assumptions.
A worked example
Worked example: a 400-mile trip in a vehicle returning 25 MPG uses about 16 gallons. At $3.50 per gallon, the fuel portion is about $56; a different economy changes both gallons and cost.
Compare the same trip and fuel price for every vehicle. Real consumption can differ from the label because of load, weather, traffic, and driving style.
Fuel cost is only one ownership input. Purchase price, maintenance, insurance, load, and the percentage of city versus highway driving can outweigh a small rating difference.
The best fuel comparison uses the same trip, price, and conditions rather than relying on an unexplained economy number.
COMMON QUESTIONS
Frequently asked questions
Is higher MPG always cheaper?
Usually for the same distance and fuel price, but purchase cost, maintenance, fuel price, and driving pattern also matter.
Why does L/100 km move in the opposite direction?
It measures fuel consumed for a fixed distance, while MPG measures distance traveled for a fixed fuel amount.