Back in the Dark Ages, when everything was carburetors and clay tablets, I was always taught something more like the five minute rule — that the cost of gas burned in starting the motor was about five minutes worth of operation. Not surprisingly, more efficient engines (and better starters) make that rule a lot smaller.
Of course, I also recall auto touring in Italy a few years back, where the diesel cars we had automatically shut themselves off when you were stopped at a traffic signal, starting as soon as you release the break and put your foot on the gas. Now that's efficiency.
If Your Car Will Be Idling for More Than 10 Seconds, Just Turn it Off
There are plenty of occasions where you might find your car idling. In a long drive through, picking someone up from work, or just sitting in your driveway. If you’re idling longer than 10 seconds, though, you’re better off turning off the engine.
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If my car idles for more than 10 seconds, the AC is blowing full blast on my face and turning the car off would kill me.
OK, would make me uncomfortable. Even here in greater Seattle as opposed to my old stomping grounds of So Cal.
Exactly. Idling keeps your accessories running without straining your battery. Also, I don't recommend anyone shut their engine off at a stop-light, even if that light is a full 2-minutes. Further, while you're saving a bit of fuel, you're wearing other things that take wear worse than the actual engine, like your battery, starter, oil systems, and several others. You may save on fuel, but you may lose a lot of that elsewhere.
If you're going to be idling for over 5-minutes and don't need the AC, that's one thing. If it's one minute to wait for someone to walk from their door to the passenger seat, leave the engine on.
The engine stopping by itself to re-start when needed took weeks to get used to when I first bought my Prius. It reminded me too much of my days driving a manual transmission and stalling out at a stop.
Of course, now that I’m driving a Prius Plug-In, I get upset if the motor starts at all during the first 15 minutes of driving each day (unless it’s cold and the heat is needed… and in NH, it does get cold). That usually means I’ve gunned the engine or done something else I should know better than doing.
If other cars had whatever starter system the Prius has, the rule would be even shorter. But, of course, they’re probably also need the electric motor to give you the initial move while the engine is starting up again…
One of the features that the auto companies are doing on new cars is an auto on/off sensor which turns the car off at idle, and restarts it when you start driving.