Fuel Economy (MPG) Calculator
The fuel economy in your car’s brochure was measured in a lab. Your real figure depends on traffic, weather, load and driving style — and the only way to know it is the fill-up method. This calculator turns distance driven and fuel used into L/100km or MPG, and converts between the two.
Track it over several fill-ups to spot trends: a rising consumption figure is often the first symptom of a mechanical problem.
Enter valid values to see the results.
Formula
The constant 235.215 comes from unit definitions (1 mile = 1.609 km, 1 US gallon = 3.785 L). Note that US MPG and UK (imperial) MPG differ: 1 UK gallon = 4.546 L, so UK figures are about 20% higher.
How to measure
- Fill the tank completely and reset the trip odometer (or note the mileage).
- Drive normally until the next fill-up, then fill to full again and note the litres or gallons added — that is the fuel used for the distance on the trip meter.
- Enter the trip distance and the fuel added. Repeat over 3–4 tanks for a reliable average.
Worked example
Practical tips
- Always fill to the same “first click” of the pump nozzle for consistent measurements.
- Winter consumption is typically 10–20% higher: cold engines, winter fuel blends and heater loads.
- A sudden consumption increase can indicate under-inflated tires, a dirty air filter or a failing oxygen sensor.
- Use the figure from this calculator in the Fuel Cost Calculator for realistic trip budgets.
L/100km to MPG (US) conversion reference
| L/100km | MPG (US) | MPG (UK) |
|---|---|---|
| 4.0 | 58.8 | 70.6 |
| 6.0 | 39.2 | 47.1 |
| 8.0 | 29.4 | 35.3 |
| 10.0 | 23.5 | 28.2 |
| 12.0 | 19.6 | 23.5 |
Common mistakes
- 1 Measuring over a single short trip — one tank is noisy; average several.
- 2 Comparing US MPG with UK MPG figures as if they were the same unit.
- 3 Trusting the dashboard computer blindly — many read 5–10% optimistic.
Frequently asked questions
How do I calculate MPG from a fill-up?
Divide the miles driven since the last fill by the gallons just added. Example: 323 miles on 11.1 gallons = 29.1 MPG.
How do I convert L/100km to MPG?
MPG (US) = 235.215 ÷ L/100km. So 8 L/100km ≈ 29.4 MPG, and 5 L/100km ≈ 47 MPG.
Why is my real consumption worse than the official figure?
Official figures come from standardized lab cycles. Cold starts, traffic, hills, cargo and speeds above 100 km/h all push real-world use 15–25% higher.
Is lower L/100km better, or higher MPG?
Both mean better economy: L/100km measures fuel per distance (lower wins), MPG measures distance per fuel (higher wins).