How this fuel cost calculator works
Trip cost is fuel used multiplied by pump price. Fuel used is distance divided by economy. Every economy unit is converted to litres per 100 km first, using 1 mile = 1.609344 km, 1 US gallon = 3.785 L, and 1 UK gallon = 4.546 L, so you can mix miles with a per-litre price.
The fill-up mode uses MotorLog's rule exactly: economy is only measured between two full fills, and the volume for the interval is this fill plus any partial fills since the previous full one. That is why a single fill-up cannot give an MPG figure.
Sources
Formulas copied from MotorLog's fuel economy engine on September 17, 2026: mpg = miles / volumeUsed and costPerMile = costUsed / miles, where volumeUsed is the closing full fill plus banked partial fills.
Questions
How do I calculate the fuel cost of a trip?
Divide the distance by your fuel economy to get the fuel needed, then multiply by the price per gallon or litre. For 250 miles at 32 MPG and $3.45 per gallon: 250 ÷ 32 = 7.81 gallons, × 3.45 = $26.95.
How do I work out MPG from fill-ups?
Fill the tank, note the odometer, drive, fill it again to full. MPG = miles driven ÷ gallons added on the second fill. Any partial fills in between are added to that gallon figure. Two full fills are the minimum.
What is cost per mile?
Total spent on fuel divided by miles driven over the same interval. MotorLog also adds service costs to give an all-in cost per mile in its Insights view.
Why does the calculator support L/100 km, km/L, and UK MPG?
Fuel economy is quoted differently by region. All inputs are converted to litres per 100 km internally, so mixing units such as miles with litre prices works.
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