除非是很老的車型，無論自排手排，空檔滑行不但更費油，而且危險。

空檔滑行的時候，引擎維持怠速運轉，一般是每分鐘800轉左右，必須燒油，而且打空檔等於放棄引擎制動力，有狀況難以應變，運氣不好的時候會付出生命代價。

掛D檔滑行的時候肯定是減速狀態，現在的汽車都不用化油器，改用燃料噴射系統，在減速的時候根本不噴油，不消耗燃料，肯定比較省油。

下面是美國科技媒體的分析：

Coasting in Neutral Does Not Save Gas: Claim Check

Coasting downhill in neutral consumes less fuel than in gear, right? Wrong. Coasting in neutral is dangerous andit burns up more fuel. Here"s why.By Mike AllenJul 29, 2010I 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.

除了陡坡長坡掛擋滑行比較安全，其它短距離滑行是省油的，開過手動擋才有發言權，其它的不懂裝懂人太多，市區油耗5.3升，經常滑行

空擋滑行不省油反而費油主要指現在的自動擋汽車，不提倡空擋滑行也主要指的自動擋汽車。從安全形度出發，手動擋車輛也不提倡空擋滑行。和過去的機械操控方式不同，今天汽車的許多操控都是依靠助力輔助來完成，比如剎車系統和轉向系統，這些系統在車輛熄火後儘管仍然發揮作用，但是效率要大大降低，手動擋車輛不提倡空擋滑行就是害怕在行駛過程中汽車意外熄火，一些依靠發動機運轉帶動的助力系統失效而發生交通事故。

自動擋車輛滑行為什麼費油？

和過去使用化油器供油的車輛不同，今天的汽車都是由ECU自動控制噴油嘴的噴油量，自動擋汽車在空擋滑行的時候，發動機是處在怠速的形態工作，噴油嘴會持續給發動機供油以維持發動機的怠速運轉。而帶檔行駛時，車輛ECU會透過各種感應器偵測到車輛當前的行駛狀態，就會向燃油系統下達停止供油的指令。在怠速供油和停止供油之間，當然是停止供油更省油，所以自動擋汽車帶檔滑行更省油。

很多媒體都說帶檔滑行發動機不噴油，時間長了大家都這樣認為了。實際上這個說法是不對的，帶檔滑行仍然噴油，這事不抬槓，找個帶行車電腦的車看看就知道了。

你好！空檔滑行為什麼不省油，以前的化油器車空檔滑行的確可以省油，電噴車則不一定省油，因為有些車在加速中鬆開油門，這個時候噴油系統停止供油，當發動機轉速轉過低再加油門時恢復供油，所以說電噴車帶檔滑行比空檔滑行更省油又安全。

為什麼空檔滑行不安全？空檔滑行減速僅以剎車來控制車速，這種情況下會存在危險隱患，長時間剎車，剎車片和剎車盤，會因為過熱而剎車效果變差嚴重會導致失靈，導致不必要的事故發生。

怎麼做比較合適？正確做法是根據實際車速掛上合適的檔位，讓發動機提高轉速起到輔助制動效果，這樣又安全又延長剎車的使用壽命。