Coasting is the process of driving a car downhill with either the clutch depressed or the gear stick in neutral - or both at the same time. The effect of this is that your wheels are disengaged from the engine, meaning engine braking (using gears to slow down) is not possible. Coasting means the car is taken down an incline by gravity and the vehicle's momentum.
Some drivers choose to coast because it disengages the engine; the idea is that coasting will reduce fuel consumption - but this is incorrect.
- You pick up speed quicker and go faster than you otherwise would.
- You have less control because you can’t control speed via engine braking - since the engine is not connected to the wheels.
- Because you are going faster, you have to use your mechanical brakes more aggressively, which can wear out brakes more quickly.
- It becomes more challenging to take a safe, steady line around a corner - meaning you'll have less control if you cross the path of another road user or other hazard.
- If you do need to re-engage the gears in order to react to a hazard, you'll have less time to do so than if you weren’t coasting.
While it is not illegal to coast downhill, if you get into an accident and you are deemed not to be fully in control of your vehicle, you would have committed an offence.
Modern cars are fitted with Electronic Control Units (ECUs). With the engine engaged, these devices reduce fuel use when the accelerator is not depressed - e.g. when you are going downhill.
But what if you coast down a hill in neutral? This disconnects the engine from the wheels which, with your foot off the accelerator, means the car cannot get the rotational power it needs from the wheel. In this case, it uses a small amount of fuel to keep engine powered - essentially "idling" - rather than drawing the necessary power from the wheels.
While with older cars coasting could save a small amount of fuel, with modern ECUs this technique does not work.
But whether you have an older or a newer car, coasting is potentially dangerous and should be avoided.
Further reading on the dangers of coasting is available in the Highway Code (Rule 122):