Coasting


#1

In my daily commute, I come across a stretch of road about 3/4 of, to a mile in length. I usually put my standard transmission in neutral and coast. In my attempt at being frugal on gas, might I actually be doing more harm to my car just to save a few cents?


#2

You’re not saving any gas at all. Not one penny’s worth.

In addition, coasting in neutral can be VERY DANGEROUS.

Stop coasting in neutral.

If you search this topic you fill find TONS of information on why coasting is NEVER a good idea. It’s too much information to list here.

No coasting in neutral. Stop doing it before something bad happens.


#3

Though generally it’s not recommended, IMO with a manual, not auto, it really depends upon the grade and how much speed you gather. I see no problem if you’re not frequently braking and your car stays within the speed limits w/o braking. At speeds in the 35 to 45 mph range, my autos in OD do little to brake regardless.


#4

You are wasting gas, not saving it. You are creating a dangerous situation. Let’s say your engine stalls (it happens). You have no power steering or power brakes. You will crash. Your tombstone will read:
“Here Lies GoGo57. Might have saved a few cents”


#5

If your car is less than 20 years old, then it wouldn’t consume any gas at all while going down the hill in gear (the weight of the car turns the engine so no gas is needed). However, when you take it out of gear the computer has to send fuel to the engine to keep it idling.


#6

I saw a thread on this about a week ago. I know very little about cars so please forgive me for the repetitive question - is a car going down hill actually get no gas at all or very little? I thought the engine needed at least some gas to run/avoid stalling out.


#7

Many (most? all?) cars with automatic transmission have something called deceleration fuel cut off (DFCO) that shuts off gas to the injectors when you are coasting above a certain speed and rpms. The inertia of the car is enough to keep the engine turning (and generating electric and vacuum for your accessories and power steering and brakes) without burning gas.

I assume that manual trans also have DFCO but I don’t know for sure. Older cars with carbureators and early model fuel injected cars will not have it. You might be able to tell using a scan tool while you are driving to monitor your injectors or your instantaneous fuel mileage. Or try googling your specific car model and the term “deceleration fuel cut off”.

In neutral, your injectors will still be firing gas into the cylinders.


#8

I’m assuming we’re talking about a downhill here, and not a straight stretch of road where he’s coasting and just slowing down over the 3/4 mile.

We don’t know that he’s wasting gas. If it’s an older standard transmission it’s not cutting the fuel in gear at idle. Coasting would save gas in that situation.

Power steering is pretty irrelevant above about 10mph because the turning resistance goes down rapidly as speed increases, so that’s a needless fear.

The brakes is a bit more accurate, though really he’d have one or two applications before hydraulic pressure was lost (and it’d be pretty easy to get that pressure back with a standard - put it in gear and let out the clutch. The inertia of the vehicle will re-start the car).

That said, the main reason it isn’t a good idea to coast in neutral is because if someone is about to rear-end you, it’s nice to be in gear so you can accelerate away from them. Of course, really, if you pay attention, you’d have time to put it in a lower gear than the top gear you’d normally be in, and therefore be able to accelerate away even faster … .

Bottom line: It’s not really worth it to bother with putting it in neutral for the tiny amount of fuel you save. If you kept the car for 20 years and did this every single day you MIGHT get a little less than a tank of gas out of it, since you’d be saving fuel for a total of 273.75 miles. But provided you pay attention, which you should be doing anyway, there’s nothing inherently dangerous about the practice.


#9

is a car going down hill actually get no gas at all or very little? I thought the engine needed at least some gas to run/avoid stalling out.

On modern fuel injected cars, no fuel is used at all when the engine is being turned over faster than the set idle speed.

There is no problem with stalling out.

#10

I would suggest you search using the following… is it legal to coast in neutral … and you will get all the information you want and more…


#11

I will take the alternate of that expressed here, We have had many recommendations on this board that brakes are cheaper than transmissions, how does this differ? Coasting in neutral has no detrimental effect as far as I know. Is it legal, I have never heard it is not, conversely I have seen engine braking prohibited signs, show me one ticket for coasting in neutral. Coast away in my book. Be reminded this is a manual transmission.


#12

There should be NO extra wear on the clutch and transmission if you rev-match before you let the clutch out, like any competent driver would do when downshifting. Simply letting the clutch out after downshifting is jerky at best, and down right dangerous in the rain or snow.

And that engine braking prohibition is not meant for drivers of gasoline powered vehicles. It is meant for trucks that engine brake by making loud jackhammer sounds. And they are located in level residential area. On the other hand, I’ve routinely seen signs that says “trucks use lower gears”.


#13

Not that I condone this, but you can try it yourself. If you can shut off your engine without locking the wheel, shut off the engine while the car is heading downhill in gear. Your injectors will not be firing. Your engine should feel and sound as if the key is on.

However, do it at your own risk. If your car is new enough, both the speedometer and tachometer will not work as the computer is off and you’ll have no idea how close you’re to the engine’s redline.


#14

DFCO only comes into play when your foot is completely off the gas and the engine is being overrun by about 200 rpm over idle speed or higher. So, it depends on the situation.
If the hill is steep enough to need engine braking anyway to check the car’s speed, then you definitely want to keep it in gear and let DFCO make the car use zero gas.

If the downgrade is so mild that you end up going the intended speed without any braking, engine or otherwise, DFCO does not come into play. You will be giving the engine just enough throttle to prevent it from braking the car so the engine will let the car coast or you will let the car coast by disengaging the clutch and letting the engine idle. In this case, letting the engine idle will use less fuel.

If you leave it in gear with your foot completely off the gas, you will use zero gas but the engine braking will slow your car below your intended cruising speed. Now, the question that has to be answered is “will the gas saved in DFCO replace the kinetic energy lost by engine braking during the DFCO when I accelerate back to cruising speed?”


#15

That [key off] trick will work for the OP’s car with manual transmission. It shouldn’t be tried on a car with an automatic. Most of those are electronically shifted, and most will go to neutral when the electrical power is turned off.