How is MPG calculated from a fill-up?

Miles per gallon is simply distance travelled divided by gallons of fuel added to refill the tank. For the cleanest reading, fill the tank fully both before and after the trip so the fuel added equals the fuel actually burned.

How do you convert between MPG and L/100km?

They are reciprocal measures on different units. The tool converts your distance to kilometres and fuel to litres, then computes litres times 100 divided by kilometres. Higher MPG corresponds to lower L/100km, since L/100km measures consumption rather than efficiency.

Where does the CO2 figure come from?

Burning one US gallon of gasoline releases about 19.6 pounds, or 8.887 kilograms, of carbon dioxide, the EPA standard figure. The tool multiplies your gallons burned by 8.887 kg. Diesel produces roughly 15 percent more CO2 per gallon because it is denser.

Why measure economy per trip instead of trusting the dashboard?

Dashboard readouts estimate consumption from sensors and can drift by several percent. A measured fill-up to fill-up calculation is the ground truth, which is why it is the method used for fuel-economy comparisons and warranty disputes.

Can I track several fill-ups?

Each calculation covers one trip and fill-up. To find a rolling average, sum the total distance and total fuel across several tanks and enter those totals, which averages out the variation from how full each tank was.

What is the Fuel Economy by Trip Calculator?

Computes real-world fuel economy in MPG and L/100km from odometer start and end or a direct trip distance and the fuel added. Also calculates total fuel cost, cost per mile or kilometre, and CO2 output at 19.6 lb per gallon of gasoline. It runs free in your browser on Gera Tools, with nothing uploaded.

Fuel Economy by Trip Calculator

Name: Fuel Economy by Trip Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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The only way to know your true fuel economy is to measure it from a real fill-up. This calculator takes the distance you drove and the fuel you added, then reports MPG, L/100km, the cost of that fuel, and the carbon dioxide it produced.

How it works

Economy is computed directly from distance and volume, with unit conversions applied so both measures are shown:

US MPG    = miles / US gallons
L/100km   = litres × 100 / km
1 US gallon = 3.785411784 L      1 mile = 1.609344 km
CO₂       = gallons × 8.887 kg   (19.6 lb per gallon, gasoline)
cost      = fuel added × price per unit

Distance can come from your odometer start and end readings or from a direct trip figure, whichever you have.

Why MPG and L/100km tell the same story differently

MPG is an efficiency measure — higher is better. L/100km is a consumption measure — lower is better. They are mathematically reciprocal once unit conversions are applied, so a car that does 40 US MPG also does about 5.88 L/100km. European drivers tend to use L/100km because it scales linearly with cost: saving 1 L/100km on a 20,000 km year saves 200 litres of fuel. US drivers think in MPG, where improvements at the low end (10→15 MPG) save more fuel than identical-sounding steps at the high end (40→45 MPG). Both views are useful; this calculator shows you both at once.

Worked example

You start at odometer reading 41,820 miles, drive to 42,145 miles, and add 10.3 gallons at $3.49/gallon:

Trip distance: 42,145 − 41,820 = 325 miles
Fuel economy: 325 ÷ 10.3 = 31.55 MPG
In metric: 9.46 L/100km
Fuel cost: 10.3 × $3.49 = $35.95
CO₂ emitted: 10.3 × 8.887 kg = 91.5 kg

Compare 31.55 MPG to your vehicle’s EPA combined rating to see whether your real-world driving conditions are better or worse than the test cycle. Highway driving typically beats the rating; stop-and-go city driving typically falls below it.

Getting accurate readings

The fill-up method is the gold standard, but it requires care:

Fill to the same click both times. If the pump clicks off at different tank levels, the measured fuel is wrong.
Measure over several tanks. Short trips have more measurement noise; three to five tanks gives a stable average.
Avoid partial fills. A half-tank measurement makes the denominator fragile to small errors.
Note conditions. Extreme cold, roof cargo, towing, or mountain driving will all lower real-world economy compared to the baseline.

To find a rolling average, sum total miles and total gallons across multiple fill-ups and enter those combined totals — the math averages out the tank-to-tank variation automatically.