I understand some cars have a coasting function to save on fuel. And I hear one or two can even shut down the engine while coasting. I am thinking of saving fuel this way too. Can I shift my car's gearbox to neutral while I am cruising on the highway? Or turn off the ignition? Is it safe and will it damage my car?
If your car has an automatic transmission, you should not under any circumstance allow it to coast in neutral. Doing so will cause serious and costly premature wear to the transmission.
The cars with coasting function have gearboxes that are specially designed for the purpose.
An interesting point to note is that the dual-clutch transmission is, strictly speaking, not an automatic. Their internals are similar to a manual gearbox and hence can be manually shifted to neutral when coasting, without causing any damage.
However, this is a driving style that needs practice, as you need to be fully aware that you are in neutral and remember to shift back to Drive once your coasting is done.
Switching off the ignition is dangerous. In cars with such an engine shutdown function, ignition is re-established the moment your right foot presses on the accelerator pedal.
If you attempt to emulate this manoeuvre, you are unlikely to be quick enough to re-start the engine when you need to.
Also, in an emergency evasive manoeuvre, you may panic and forget that the engine is off.
Worst of all, when the engine is switched off, you lose steering and braking assistance, making it hard to steer and brake.
Cars with such a function have a 48-volt electrical system which kicks in when the engine is off.