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Is it safe to coast downhill while not in gear?

Coasting is where you either take your car out of gear and put it into neutral, or you push the clutch in to disengage the transmission. Pushing the clutch in only works in a manual vehicle, but it is possible to move a gearbox from drive to neutral in an automatic car.

Some people believe that coasting is better for fuel economy. Those people are wrong. It’s actually worse for your vehicle in a number of ways.

In a modern vehicle with electronic fuel injection, the engine management system means that coasting actually uses more fuel than leaving the vehicle in gear. When you coast, the engine has to burn fuel to maintain idle revs. However, if you leave it in gear, the wheels are turning the transmission which is turning the engine and it uses virtually no fuel.

You might get an advantage with an old vehicle with carburettors, but that still doesn’t mean it’s a good idea. When the vehicle is in gear and you take your foot off the accelerator, the engine’s resistance creates engine braking. To a point, this stops the vehicle from gaining speed, unless the angle of the slope overcomes the resistance of the engine, the road and wind resistance. If the vehicle is not in gear, it’s only the resistance of the wind and the road are slowing it down, and it doesn’t take much of a slope to overcome that, in which case the vehicle keeps accelerating.

To stop it from accelerating you need to use the brakes, which then risks brake fade and wears your brakes out quicker.

In some cases, you need acceleration to get out of danger. If your vehicle is not in gear, you’ll waste time putting it back in gear before you can accelerate again.

It’s common for truck drivers using a manual gearbox to coast to early when they struggle to get the gearbox into a low gear when coming to a stop.

Is coasting illegal?

Coasting isn’t illegal in New Zealand, but it definitely is not recommended as you are not in control of your vehicle. In some countries, it’s illegal for some vehicles (e.g. trucks) or all vehicles.

What happens if you get into danger when coasting?

The most likely outcome is that you will get brake fade and your brakes will fail. Then, because the wheel speed is so high, it can be difficult to get it into a meaningful gear for engine braking. Eventually, unless you can stay on the road, you’ll end up off the road with extremely hot brakes (in summer, they’ll be hot enough to start a grass fire). If the brakes have overheated, then the disks will change colour and possibly warp – telltale signs that you were coasting.

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Darren is a member of the Institute of Advanced Motorists and the NZ Motoring Writers' Guild

Posted in Advice
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