Calculation details
Planning estimate only, not financial, tax, or legal advice. Verify assumptions and current rules before making decisions.
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Use this result
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What does this calculator estimate?
A gas mileage calculator converts distance and fuel into fuel economy — MPG or liters per 100 km. Enter the distance and fuel used to see your efficiency.
- MPG = miles ÷ gallons
- L/100km = (liters ÷ km) × 100
- Higher MPG = better efficiency; lower L/100km = better
What fuel economy measures
Fuel economy tells you how efficiently a vehicle converts fuel into distance. MPG is the US convention; L/100km is the metric convention. Tracking it reveals driving habits, maintenance needs, and the true cost per mile.
Limitations to watch for
The estimate is only as accurate as your fill-up measurement — fill the tank completely, note the odometer, and drive before refilling. City vs highway driving, tire pressure, and load change real-world MPG. The tool uses US gallons; imperial gallons differ.
How to use it in practice
Fill up completely and reset the trip odometer; at the next fill, divide the miles by the gallons pumped. Track over several tanks for a stable average. Use the result to estimate trip costs and spot efficiency drops.
['Enter the distance driven.', 'Enter the fuel used.', 'Choose units (mi/gal or km/L); the tool converts.']
How this calculator works
Formula
US: MPG = miles ÷ gallons. Metric: L/100km = (liters ÷ km) × 100. Mixed units are converted (1 mile = 1.60934 km, 1 US gallon = 3.78541 L).
Worked example
300 miles on 10 gallons = 30 MPG. Or 40 liters for 500 km = 8.0 L/100km.
Assumptions to verify
- The tank was full at both ends of the measurement.
- US gallons are used for MPG.
- The driving is a representative mix.
Frequently asked questions
How do I calculate MPG?
Divide miles driven by gallons used: 300 miles ÷ 10 gallons = 30 MPG.
What is L/100km?
Liters of fuel per 100 kilometers — the metric fuel economy. 8 L/100km means the car uses 8 liters to drive 100 km.
How do I measure it accurately?
Fill the tank completely, reset the trip odometer, drive, then refill to full and divide the miles by the gallons pumped.
What's a good MPG?
It depends on the vehicle: 30+ MPG is good for a car, 15–20 for a truck or SUV. Compare against the vehicle's EPA rating.
Does driving style matter?
Yes — rapid acceleration, speeding, and idling cut MPG significantly. Smooth driving and proper tire pressure help.
What's the difference between US and imperial gallons?
An imperial gallon is larger (4.546 L vs 3.785 L). MPG figures differ between the two — the tool uses US gallons.
How do I estimate trip cost?
Divide the trip distance by your MPG and multiply by the fuel price. The fuel trip cost calculator does this directly.
Cite this tool
BoringToolsKit. “Gas Mileage Calculator.” boringtoolskit.com/gas-mileage-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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