The formula
range = litres available ÷ consumption × 100
Where it comes from
The fuel gauge is not linear: the needle falls slowly through the first half and much faster through the second, because the float moves inside a tank of irregular shape. That is why range is worth working out in litres rather than trusting the needle.
How to work it out by hand
- Work out the litres left from the level in the tank
- Subtract the reserve you want to keep
- Divide by the consumption per hundred kilometres
- Multiply by a hundred
What is worth knowing
The warning light normally comes on with 5 to 10 litres left, which is 60 to 150 km depending on the car. Even so, routinely running the reserve down is a bad idea on cars with an in-tank fuel pump: the fuel itself cools the pump, and driving nearly empty makes it work hotter.