Do you play fuel station roulette? All about when the low fuel light is on

We’ve all driven around with the low fuel light on. Some people even play fuel station roulette on a regular basis, driving as far as they dare with the orange light on and their car running on the dregs of its fuel. But do you really know how many miles of range your car has when the low fuel light comes on?

What is your car’s fuel light range?

That is like asking how long a piece of string is. It depends on how the fuel gauge is calibrated. On some cars, the gauge will tell you the tank is empty with zero range when it’s still got around a gallon left in it. On others, the opposite is true. We’ve heard tales of drivers whose cars have stopped running despite the read out saying they’ve still got a range of 30 miles.

There have recently been stories claiming to calculate a car’s range once the fuel light comes on. This uses the car’s mpg and estimates that the light comes on with 10 per cent of the tank remaining. But as every model of car has a different fuel system, this is unlikely to be accurate.

How accurate are fuel gauges?

This varies greatly. Some aren’t very accurate at all, some are; many err on the side of caution. Some modern cars have a range function on their computer. When the fuel light comes on, the range function will probably pop up to say how many miles you have left in the fuel tank. However, this is usually an average figure gleaned from your overall mpg up to that point. If you’re on a motorway where you’re typically driving quickly, your car will burn fuel at a faster rate.

Is there really a reserve fuel tank?

Some people think cars have a second ‘reserve’ fuel tank like old aeroplanes did. They don’t. When the fuel light comes on, the fuel level is towards the bottom of the tank. You’re into what’s called the reserve fuel. However when the fuel light comes on does vary. On some cars it will be when the tank has 15 per cent of its fuel remaining. On others it will be at less than 10 per cent. Check your car’s user manual to find out what your reserve fuel is.

How many people run out of fuel every year?

Estimates suggest around 900,000 people a year run out of fuel. According to Highways England, the body that looks after the UK’s motorway and A-road network, one in 12 cars that ‘breaks down’ on a major route has actually run out of fuel.

What to do when the fuel light comes on

Drivers should view the fuel light as a final warning and head to the nearest fuel station. The reason being you have some fuel left in the car but can’t be certain how much. You should try to use the accelerator sparingly so attempt to maintain momentum where it’s safe to do so. Your car’s most economical speed depends on the kind of engine it has and its gearing. But the lower the revs and the less the accelerator is used, the more mileage you’ll get. If you want to make tiny gains in mileage, keep windows up at higher speeds and turn the air-conditioning off.

Is running your car low on fuel harmful?

Cars are clever but they can’t defy the laws of physics. Car manufacturer SEAT says it’s perfectly reasonable to expect a car that’s running low on fuel to misfire or for the engine to stutter, particularly on gradients. This is because the fuel pump can’t pick up the dregs of fuel in the tank. Running a car dry can also cause unburnt fuel to enter the exhaust system. This might be harmful to the catalytic converter in petrol cars or the particulate filter in diesels. Both are very expensive components to replace.

Modern fuel injection cars use an electric fuel pump which sits in the fuel tank. This is so it can use the fuel as a coolant. Run the car out of fuel and the fuel pump will start to get hot. Experts say this should only result in damage if the car is run persistently with a very low fuel level.