Fuel is one of the biggest running costs of any car, and your fuel economy is also a surprisingly good health gauge. Here’s how to measure it properly — in whatever units you use — and why it’s worth doing.
The units, quickly
- km/L (kilometers per liter) — higher is better. Common in much of Asia.
- MPG (miles per gallon) — higher is better. Used in the US and UK (note: US and UK gallons differ).
- L/100km (liters per 100 km) — lower is better. Standard in Europe.
They all measure the same thing: how far you go per unit of fuel.
How to calculate it by hand
The reliable method takes two fill-ups:
- Fill the tank completely and reset your trip meter (or note the odometer).
- Drive normally until you’re ready to refuel — ideally most of a tank.
- Fill up completely again and note the fuel added and the distance since step 1.
- Do the math:
- km/L = distance in km ÷ liters added
- MPG = miles driven ÷ gallons added
- L/100km = (liters added ÷ km driven) × 100
One tank gives a rough figure; averaging several tanks smooths out variation from traffic, terrain, and driving style, and gives you a trustworthy baseline.
Why not just trust the dashboard?
Built-in fuel economy displays are convenient but estimated, and they tend to read optimistically. If you want a number you can actually budget and diagnose with, calculate it from fuel you really bought and distance you really drove.
The real payoff: early warning
Once you know your car’s normal figure, a change becomes meaningful. A steady drop of 10–15% usually isn’t your imagination — it commonly points to:
- Under-inflated tires
- A clogged air filter (also on the maintenance schedule)
- Worn spark plugs or a lazy oxygen sensor
- Dragging brakes
- Overdue oil (see how often to change your oil)
Catching that early turns a slow money leak into a cheap fix.
Make tracking effortless
Doing the math by hand at every fill-up gets old fast. The fuel & mileage tracking feature is built for exactly this: enter the fuel, price, and odometer, and Kotselog computes your km/L or MPG for that tank, updates your averages and cost per kilometer, and plots the trend so a real dip jumps out. It even works offline at the pump. Open the app and log your first tank tonight.
Your fuel figure also feeds your cost per month and cost per km. Want to move the number, not just watch it? See 10 driving habits that actually improve your fuel economy. Driving in the Philippines? See fuel economy tips + tracking km/L.
Frequently asked questions
How do I calculate my car's fuel economy?
Fill the tank, reset your trip meter, drive normally, then fill up again. Divide the distance driven by the fuel added: distance ÷ liters = km/L, or miles ÷ gallons = MPG. For L/100km, divide liters by distance and multiply by 100. Averaging several tanks is more accurate than one.
Why is my dashboard MPG different from my real MPG?
On-board fuel economy displays are estimates and are often optimistic by several percent. A figure calculated from actual fuel purchased and distance driven is the honest number.
What's a sudden drop in fuel economy a sign of?
Commonly under-inflated tires, a dirty air filter, worn spark plugs, a failing sensor, dragging brakes, or simply heavier driving. A consistent drop is worth investigating — it often points to a cheap fix before it becomes an expensive one.
How Kotselog helps: Log fill-ups in seconds and see your real mileage — km/L or MPG — over time. See the fuel economy feature →
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