空擋滑行發動機轉速低，同樣的油行駛更遠距離，為什麼有人說空擋不省油反而費？

I get mail. I've said, on the record, many times, that it's a bad idea to coast downhill or up to a stop sign in neutral. It's unsafe. You need to be able to use the accelerator to avoid an unexpected road hazard; cars don't handle well in neutral during sharp cornering maneuvers when the engine isn't connected to the drivetrain.

So why on earth would you put the transmission in neutral—whether on manual or automatic—when coasting? Apparently there are a lot of people out there who think they are saving gas by doing so. They're wrong.

Here's the argument I get: "The engine is idling while coasting, so no less gas is used with the transmission engaged and the accelerator let up."

When coasting in neutral, the engine is idling, consuming just as much gasoline as when it's idling at a traffic light or warming up in your driveway, roughly gallons per hour (gph), depending on your vehicle. Some small cars with roller cams may do slightly better, but a rule of thumb for idling fuel consumption is 1 gph . Let's use 1 gph as a starting point, just to make the math simpler.

Suppose you are coasting down a mile-long hill at an average speed of 30 mph, which will take close to 2 minutes. During this period, you'll consume approximately 0.033 gallons of gasoline, for a consumption of 30 mpg.

I've replicated these conditions with instrumented cars, both with scan tools and with an oscilloscope, measuring the leads leading into the fuel injectors. The signal controlling the injector is a 12-volt square wave. It's pulse-width-modulated, varying from 5 percent or so at idle to around 80 percent or so at full throttle. The higher the percentage of on time to off time, the more fuel. There's one on pulse for every cylinder firing, so the consumption also varies with engine speed (rpm). All vehicles show a short pulse width at idle, regardless of whether they're sitting in traffic at a red light or coasting downhill—at idle—in neutral. (Actually, they use a fraction more fuel sitting in drive at a traffic light, because of the drag in the torque converter, but I digress).

Almost all vehicles show a pulse width of zero when coasting while in gear. Zero, as in there is no fuel injected at all. Yes, the engine is turning over, the pistons are going up and down, the water pump, alternator and a/c compressor are working, so technically you can say the engine is running, sort of. But it's not consuming any fuel. And that goes for automatic or manuals.

Okay, eventually, at the bottom of the hill or as you creep up to the traffic light, the engine finally will slow to idle rpm—at which point the fuel injection will wake up and start adding fuel to keep the engine from stalling. That usually starts at around 1000 rpm, and if you pay attention, you can sense when it's happening as the engine will rev up slightly. And that's when the scan tool or oscilloscope will show injector dwell rise from 0 to 5 to 10 percent. So you're actually wasting gas by putting your car into neutral.

I hear this argument as well: My car-mileage-information computer goes wild with increased mileage while coasting.

The algorithm the trip computer uses is not based on how much fuel is actually consumed, but on some calculated value based on airflow past the mass airflow sensor, manifold vacuum and engine rpm. And it's not accurate under these coasting conditions. That's why when we report fuel economy here at PM, we never just print the numbers we read off the trip computer's display: We use the gallons pumped into the tank divided by the mileage on the odometer—which we check against a handheld GPS.

I use a Scangauge II for a lot of diagnostics and general tinkering. It's a great tool—but I've learned not to trust the economy or gallons-used function too closely. That's why the Scangauge has a function that allows you to tell it what the engine-idle cutoff is for your particular car to get somewhere closer to the truth.

Bottom line: Don't coast in neutral. It's dangerous and won't save fuel. Turning off the key at traffic lights might.