Real fuel economy calculator
Fuel put in and distance covered. That is all it takes to know what your car really does, rather than what the trip computer claims.
Your figures
Result
- Distance per unit of fuel (km/l)
- 15.5
- Cost per 100 ($/100 km)
- $10.00
- Cost per unit of distance ($/km)
- $0.100
- Range on a full tank (km)
- 775
- Cost to fill the tank
- $77.50
- What this trip cost
- $62.00
What we assume
- The calculation assumes you filled the tank to the same point both times. It is the only way the fuel you put in matches the distance you covered.
- The range is theoretical: nobody should run a tank all the way down.
- One fill is a snapshot of one kind of driving. For a reliable average, repeat it a few times.
How it is calculated
Fuel economy is a simple ratio between fuel used and distance covered. What matters is not the formula but the method. For the figure to mean anything:
- Fill the tank until the pump clicks off and reset the trip meter.
- Drive normally until the tank is fairly low.
- Fill it to the same point and note the fuel and the distance.
Why it does not match the trip computer
Trip computers tend to be optimistic, commonly off by 3% to 8% in the flattering direction. This method is more reliable because it measures fuel you actually paid for rather than an electronic estimate.
A note on units
Switch the unit system in the preferences menu and every field and result converts: gallons and miles per gallon, or litres and litres per 100 km. The fuel price converts too, from price per litre to price per gallon.
An example
Putting in 10.6 gallons after covering 385 miles gives real economy of 36.5 mpg. At 5.87 a gallon, every 100 miles costs about 16 and the whole trip cost around 62.
Frequently asked questions
Why does my car use more than the official figure?
Because official figures are measured in controlled conditions. Real consumption depends on speed, traffic, air conditioning, load, tyre pressure and gradient. A gap of 10% to 25% is normal.
How far do I need to drive for this to be reliable?
The further the better. Under about 60 miles the margin of error is high, because the exact fill point matters too much. A nearly full tank is ideal.
How do I convert between mpg and litres per 100 km?
They are inversely related, not proportional: fewer litres per 100 km means more miles per gallon. We show both so you never have to do it yourself.
Keep calculating
All tools →- Cost per unit of distance of your car Fuel is the part you notice and the smallest part of the bill. This adds the ones you do not: insurance, tax, servicing and, above all, depreciation.
- Compare the fuel cost of two cars Two cars, two consumption figures, two fuel prices. What the difference actually adds up to over the years you will keep it.
- Electric car charging cost What a charge costs and how far it takes you. Including the energy you pay for that never reaches the battery, which nobody mentions.
- What a trip costs in fuel Distance, consumption and fuel price. What the trip really costs, tolls included, and whether the train was cheaper after all.
Updated on 2026-08-26. Calculations run in your browser; nothing you type is sent to a server.