What this calculator works out
This calculator converts litres of fuel into miles of range at a given economy figure, holding back a reserve percentage.
The reserve is the useful part. Running a tank to genuinely empty risks drawing sediment into the fuel system, and on a diesel it can introduce air that requires bleeding.
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Before you rely on the figure
- Use your own observed economy, not the manufacturer's figure. Real-world consumption is routinely 10 to 20% worse than the official test result.
- Economy varies enormously with speed, load and conditions. Motorway driving at 70 mph uses considerably more than at 60.
- A 10% reserve on a typical tank is around 5 litres, or 40 to 50 miles. That is a sensible margin, not a generous one.
- Fuel gauges are not linear. The last quarter frequently disappears faster than the first.
A UK gallon is 4.546 litres; a US gallon is 3.785. The same car is quoted at roughly 20% higher mpg in UK figures than in US ones. Comparing a UK economy figure against an American review, or entering one into a tool expecting the other, produces a 20% error in the same direction every time.
How this calculator works
Litres to gallons to miles, less the reserve:
Miles = litres × mpg ÷ 4.54609 × (1 − reserve ÷ 100)
4.54609 is the number of litres in a UK gallonDividing by 4.54609 converts litres into UK gallons, which the mpg figure then turns into miles. For a car quoted in litres per 100 km, the conversion is different: mpg equals 282.5 divided by the l/100km figure.
Worked example: 40 litres at 45 mpg
Using the default figures — 40 litres of usable fuel, 45 mpg, 10% reserve:
- Range before reserve: 396 miles
- Usable range: 356 miles
Drop the economy to 38 mpg, which is a realistic motorway figure for a car that manages 45 in mixed driving, and the usable range falls to 301 miles. That 55 mile difference is exactly the margin that matters when planning whether a journey needs a fuel stop — and it is why using an observed figure rather than the brochure one is worth the effort.
Common mistakes
- Using the manufacturer's economy figure. Real-world consumption is consistently worse.
- Confusing UK and US gallons. They differ by about 20%.
- Entering tank capacity as fuel available. A tank is rarely completely full or completely usable.
- Setting no reserve. Running to empty risks the fuel system and leaves no margin for diversions.
- Applying town economy to a motorway journey. Consumption rises sharply with speed.
Frequently asked questions
How do I measure my actual economy?
Fill the tank, note the mileage, drive normally, fill again and record the litres. Divide the miles by the litres and multiply by 4.546 for UK mpg. Repeating this over a few tanks gives a far better figure than the trip computer, which is often optimistic by several per cent.
Why is real economy worse than the official figure?
Official figures come from a standardised laboratory test, which is repeatable but does not reflect real driving. The current WLTP procedure is considerably closer to reality than the older NEDC test it replaced, but a gap remains — driving style, load, weather, tyre pressures and short journeys all push consumption up.
Is it bad to run the tank low?
It carries some risk. Sediment settles in the bottom of a tank and can be drawn into the fuel system; on older cars the in-tank fuel pump relies on fuel for cooling. Running a diesel completely dry can introduce air into the system, which on many cars needs bleeding before it will restart. Keeping a quarter tank avoids all of it.
How much does speed affect economy?
Substantially above about 55 to 60 mph, because aerodynamic drag rises with the square of speed. The often-quoted comparison is that driving at 70 mph uses around 10 to 15% more fuel than 60 mph over the same distance, and 80 mph considerably more again.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — official fuel consumption figures, WLTP testing and vehicle information
- MoneyHelper — guidance on the running costs of car ownership
Sources are checked at publication and can change — how I choose and check references.