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MPG Calculator
Fill your tank, reset the trip meter, drive normally, then fill up again. Enter the miles driven and the gallons you added to see your real fuel economy.
How to measure your real MPG
- Fill the tank completely and reset your trip odometer.
- Drive as usual until the tank is at least half empty.
- Fill up again and note the gallons (or liters) added.
- Divide miles driven by gallons added.
Fuel economy formulas
MPG = miles ÷ gallons
L/100 km = 235.215 ÷ MPG
UK MPG = US MPG × 1.201 (UK gallons are larger)
Repeat over several tanks for a more reliable average — one fill-up can be off if the pump clicked off early.
Why your MPG is lower than the window sticker
- Short trips and cold weather
- High highway speeds and aggressive acceleration
- Under-inflated tires and extra weight
- Idling and running the AC hard
Frequently asked questions
How do you calculate MPG?
Divide the miles you drove by the gallons of fuel used. 320 miles ÷ 11.5 gallons = 27.8 MPG.
How do I convert MPG to L/100 km?
Divide 235.215 by the MPG. 30 MPG ≈ 7.84 L/100 km.
What is the difference between US and UK MPG?
A UK (imperial) gallon is about 20% larger than a US gallon, so the same car shows a higher MPG number in the UK.
Sources
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