Fuel Economy Calculator
Enter a distance and the fuel used to get mpg, L/100km, and km/L together — the fastest way to compare vehicles across metric and imperial figures.
Fuel economy
mpg is miles ÷ gallons; L/100km is litres ÷ km × 100; km/L is km ÷ litres. Note that lower L/100km means better economy, while higher mpg and km/L mean better economy.
One number, three ways to read it
Fleet paperwork rarely agrees on units. A manufacturer quotes L/100km, an older logbook is in miles per gallon, and a driver talks in km per litre. This calculator takes a single measured distance and the fuel used and returns all three at once, so you can line a spec sheet up against a fuel log without reaching for a conversion table.
Measured consistently — same fill cut-off, real odometer distance, averaged over a few tanks — it is also one of the cheapest condition monitors you have. Log economy per vehicle and a quiet decline stands out long before it becomes a breakdown.
Frequently Asked Questions
How do you calculate fuel economy?
From the distance travelled and the fuel used between two brim-full fill-ups. Miles per gallon is miles divided by US gallons; L/100km is litres divided by kilometres, times 100; km/L is kilometres divided by litres. The tool works out all three from whatever units you enter.
Why does a higher L/100km mean worse economy but a higher mpg mean better?
L/100km measures fuel consumed to cover a fixed distance, so a smaller number is more efficient. mpg and km/L measure distance covered per unit of fuel, so a larger number is more efficient. That is why the same vehicle can look like it is getting 'worse' or 'better' depending on which figure you read — the tool shows all three to avoid the confusion.
How do 30 mpg and 7.84 L/100km relate?
They describe the same efficiency. 30 US mpg converts to about 7.84 L/100km and roughly 12.75 km/L. Seeing the equivalents together makes it easy to compare a metric spec sheet with an imperial fuel log.
What is the most accurate way to measure it?
Fill the tank to the same cut-off point at the start and end of a measured stretch, note the odometer distance, and record the litres or gallons added at the second fill. A single tank can be noisy, so average several fills for a figure you can trust.
Can I use this to catch a developing fault?
Yes — a steady drop in economy on a vehicle that used to return a consistent figure is a common early sign of dragging brakes, low tyre pressure, a clogged filter, or an injector problem. Tracking economy per vehicle over time turns this tool into a simple health check.
Estimates for planning — not professional mechanical, safety, or regulatory advice; follow the manufacturer schedule and local PSV/DOT rules.