What Is Fuel Economy?
Fuel economy measures how far a vehicle travels on a given amount of fuel. The most common measure in the US is miles per gallon (MPG): the higher the number, the more efficient the vehicle. This calculator turns a simple trip log — the miles you drove and the gallons you burned — into MPG, and also into the metric units L/100km and km/L used in most of the world.
How to Use It
Enter the distance you traveled in miles and the amount of fuel you used in US gallons. The easiest way to gather these numbers is the "full-tank" method: fill the tank, reset the trip odometer, drive, then refill. The miles on the trip meter are your distance, and the gallons it takes to top off the tank are your fuel used.
The Formula Explained
The core formula is $$\text{MPG} = \frac{\text{Distance (miles)}}{\text{Fuel (gallons)}}$$. To convert to liters per 100 kilometers we change gallons to liters (\(\times 3.7854\)) and miles to kilometers (\(\times 1.6093\)), then scale to a 100 km basis. Note that L/100km is an inverse measure — a lower value is better — whereas higher MPG and km/L are better.
Worked Example
Suppose you drove 300 miles and used 12 gallons of gas. $$\text{MPG} = 300 \div 12 = 25 \text{ MPG}$$ In metric terms that is about 9.41 L/100km and roughly 10.63 km/L.
FAQ
Does this use US or Imperial gallons? The calculator assumes US gallons (3.7854 liters). An Imperial gallon is larger (4.546 L), so UK figures would differ.
Why is my MPG lower than the sticker rating? Real-world MPG varies with driving style, traffic, weather, terrain, and load — city driving and high speeds reduce economy.
How can I improve fuel economy? Keep tires properly inflated, remove excess weight, avoid hard acceleration, and maintain steady highway speeds.
