Coasting or neutral for max mpgs?

[ElGato]

I've often wondered this, and have heard both sides of the argument, but I'm curious about SVTP's opinions.


-You're decelerating. It could be downhill, uphill, on flat, whatever.
-You can either:
A) take your foot off the gas with the transmission in gear or
B) put the transmission in neutral and let the motor idle.

What yields the better gas mileage. (and why)


now for the bonus question:
what situation is better for the motor? (and why)

 

[Coiled03]

The answer to both is the same: do both. Put it in neutral, AND take you foot off the gas, unless you want to bounce off the rev limiter in short order.

Ultimately, when you're decelerating, the slower you rev the engine, the less gas you burn.

For the bonus, compression braking is harder on an engine than using the brake pads to stop. Therefore, like I said, putting it in neutral is better.

 

[ElGato]

The answer to both is the same: do both. Put it in neutral, AND take you foot off the gas, unless you want to bounce off the rev limiter in short order.

Ultimately, when you're decelerating, the slower you rev the engine, the less gas you burn.

For the bonus, compression braking is harder on an engine than using the brake pads to stop. Therefore, like I said, putting it in neutral is better.

Excellent point! I will now edit the above post to reflect something I took for granted.

 

[exdeath]

The technical answer is that coasting in neutral consumes more fuel.

Coasting in gear with foot off the throttle maintains a "throttle == 0 && RPM > idle_target" condition which causes the computer to remain in deceleration fuel cut mode; the engine RPM is being maintained by the wheels and no combustion is occurring.

Neutral coasting causes the engine to dashpot to idle quickly and resume firing the injectors to stay above the idle threshold.

 

[FX4 SAPPER]

^ ive heard that as well, but i still do it. I go down a few hills where i live and it seems like it works?

 

[HYBRED]

I've often wondered this, and have heard both sides of the argument, but I'm curious about SVTP's opinions.


-You're decelerating. It could be downhill, uphill, on flat, whatever.
-You can either:
A) take your foot off the gas with the transmission in gear or
B) put the transmission in neutral and let the motor idle.

What yields the better gas mileage. (and why)


now for the bonus question:
what situation is better for the motor? (and why)

You're not one of those damned hypermilers are you? :dw:

 

[SaleenFiend]

So when is this going to be on myth busters?

 

[exdeath]

So when is this going to be on myth busters?

It doesn't have to be. It's called DFCO and has been implemented by Ford since EEC IV, and even earlier.

Whether or not you'll notice the fuel savings however is another story, as idle consumes so little to begin with.

 

[CalcVictim]

The technical answer is that coasting in neutral consumes more fuel.

Coasting in gear with foot off the throttle maintains a "throttle == 0 && RPM > idle_target" condition which causes the computer to remain in deceleration fuel cut mode; the engine RPM is being maintained by the wheels and no combustion is occurring.

Neutral coasting causes the engine to dashpot to idle quickly and resume firing the injectors to stay above the idle threshold.

This is true and was confirmed on Subaru's using a system that can monitor injector duty cycles.


I can't say with certainty that it works the same way on all cars because some might use different algorithms for fuel mapping but it certainly makes sense.

 

[Sniperdog]

put in neutral, turn off key enough to cut off engine but maintain steering and turn key back to the on position to pop clutch when coming to a stop :thumbsup:

 

[yellow03cobra]

if you are really concerned about mpg, buy a prius(I own one)
if you own a prius and are still concerned about your mpg, get a life.

when going down a hill in the prius, the gas engine shuts off, the engine braking itself is what is causing the mpg drop.

 

[Sniperdog]

Good point.,., thats why my wife drives an accord hybrid (home health nurse):-D

 

[Riskeman]

actually coasting in neutral will give better mpg. What your forgetting about is that while you may be using more fuel to idle(very very little) you are traveling much farther under no load while coasting. With the engine braking the car you will slow down much faster and not be able to take advantage of the free distance. Miles / gallon.

 

[ElGato]

You're not one of those damned hypermilers are you? :dw:

I'm suprised as an engineer that you don't have an answer.oke::lol1: Is it because you haven't thought about the question, or you just don't think people who love performance cars should be able to get good mileage?

if you are really concerned about mpg, buy a prius(I own one)
if you own a prius and are still concerned about your mpg, get a life.
.

When you get 13mpg highway in a car you love to drive, you begin to wonder things just for the sake of wondering.

 

[CalcVictim]

if you are really concerned about mpg, buy a prius(I own one)
if you own a prius and are still concerned about your mpg, get a life.

when going down a hill in the prius, the gas engine shuts off, the engine braking itself is what is causing the mpg drop.

your prius works on a different principle, the same logic doesn't apply...your engine shuts off and your battery charges when you're decelerating

actually coasting in neutral will give better mpg. What your forgetting about is that while you may be using more fuel to idle(very very little) you are traveling much farther under no load while coasting. With the engine braking the car you will slow down much faster and not be able to take advantage of the free distance. Miles / gallon.

no one is forgetting that but rather neglecting it because that is not the point, if you were coasting without the use of brakes at all then yes more likely you would get further in neutral rather then engine breaking but that is irrelevant. In reality people still use their brakes before stopping so distance is not the limiting factor.

 

[NO-BlkLightning2K]

Good point.,., thats why my wife drives an accord hybrid (home health nurse):-D

diesels and hybrids = waste of money, imo. btw, diesels only apply to this if you're only referring to fuel economy.

of course, your wife may be an exception. it's probable, given her job, that she drives enough to eventually offset the increased price of the car.

 

[Sniperdog]

diesels and hybrids = waste of money, imo. btw, diesels only apply to this if you're only referring to fuel economy.

of course, your wife may be an exception. it's probable, given her job, that she drives enough to eventually offset the increased price of the car.

word.,., its an 05 we got new late 05 she just rolled over 100k
plus I work at the dealership so that helps too:thumbsup:

 

[swoosh_stang]

I've actually killed the engine on long coasts (down hill) before, keep in mind I'm talking about a 5-6% grade for over a mile on a freeway.

I still think putting the car in nuetral and coasting would be more efficient than compression braking. Although theoretically you could hold the revs below idle if you were in top gear and going slow enough.

 

[HYBRED]

I'm suprised as an engineer that you don't have an answer.oke::lol1: Is it because you haven't thought about the question, or you just don't think people who love performance cars should be able to get good mileage?

The question has been answered well by exdeath. FWIW, I get 14.5/17 in the truck and 17/24 in the Mustang :lol1:

 

[CalcVictim]

I've actually killed the engine on long coasts (down hill) before, keep in mind I'm talking about a 5-6% grade for over a mile on a freeway.

I still think putting the car in nuetral and coasting would be more efficient than compression braking. Although theoretically you could hold the revs below idle if you were in top gear and going slow enough.

the RPM doesn't matter, you can be going downhill in gear revving at 3k rpm and you still will not use any gas......the gas used is not only a function of RPM but also a function of load on the engine