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Improving gas mileage... coasting in gear uses less gas than coasting in neutral?

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Crucial

Diamond Member
Dec 21, 2000
5,026
0
71
Do you drive on a lot of hills? I have only one hill on the freeway system where I can coast and actually pick up speed. Every other time I am maintaining a constant speed and coasting would do nothing to but slow me and the people behind me down. If I am coming to a stop I am downshiftng and don't need to coast.

I drive a 6 speed and when I put it into neutral at speed the idle is higher than when stopped.
 

5LiterMustang

Senior member
Dec 8, 2002
531
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0
Originally posted by: HardcoreRobot
cruise control when i have used it kept my speed constant, not the rpm. meaning, going up a hill, its gonna increase rpm to maintain speed
Uhhh negative...you dont increase rpm to maintain speed...2000rpm downhill is the same speed as 2000rpm up hill assuming you're using the same gear. If you're using cruise control then it will vary the amount of throttle, increasing the injector pulse or increasing the amount of fuel the injectors are firing. The tires are tied to the axle which is tied to the flywheel, which is tied to the motor, 2000rpm in 6th gear will always be 88mph in my car, whether its going up or down a hill
 

5LiterMustang

Senior member
Dec 8, 2002
531
0
0
Originally posted by: spidey07
doesn't seem right, but I'll take a stab at it.

Modern computers in a car could be sending less gas to the motor while coasting in gear, possibly even less than idle. engine RPM and rate of fuel aren't necessarily closely related - its all about the throttle and ECU.

THat's correct, throttle position and gear selection tell the ecu how much fuel to pump into the motor.
 

mk52

Senior member
Aug 8, 2000
810
0
0
Originally posted by: 5LiterMustang
Originally posted by: HardcoreRobot
cruise control when i have used it kept my speed constant, not the rpm. meaning, going up a hill, its gonna increase rpm to maintain speed
Uhhh negative...you dont increase rpm to maintain speed...2000rpm downhill is the same speed as 2000rpm up hill assuming you're using the same gear. If you're using cruise control then it will vary the amount of throttle, increasing the injector pulse or increasing the amount of fuel the injectors are firing. The tires are tied to the axle which is tied to the flywheel, which is tied to the motor, 2000rpm in 6th gear will always be 88mph in my car, whether its going up or down a hill
So you are saying that no matter whether it's a flat surface or a 40% incline you are going to go 88mph at 2000rpm??

CC on my car certainly adjusts rpm depending on whether I am going up or downhill.
 

Vic

Elite Member
Jun 12, 2001
48,686
9,872
136
Originally posted by: psteng19
Originally posted by: Pacfanweb
However, on a fuel injected vehicle, this isn't necessarily the case. Most cars nowadays, if you coast for a few seconds, will completely shut off the fuel. So they aren't burning any gas, which is obviously less than the minute amount that it burns at idle.
OK, so if you coast in gear and the computer shuts off the injectors, what keeps the engine turning?
The wheels (via momentum)?
Do the sparks fire?
The kinetic energy of the vehicle moving forward, transferred to the engine through the transmission from the wheels rolling on the ground.
 

Crucial

Diamond Member
Dec 21, 2000
5,026
0
71
Originally posted by: MelikK
Originally posted by: 5LiterMustang
Originally posted by: HardcoreRobot
cruise control when i have used it kept my speed constant, not the rpm. meaning, going up a hill, its gonna increase rpm to maintain speed
Uhhh negative...you dont increase rpm to maintain speed...2000rpm downhill is the same speed as 2000rpm up hill assuming you're using the same gear. If you're using cruise control then it will vary the amount of throttle, increasing the injector pulse or increasing the amount of fuel the injectors are firing. The tires are tied to the axle which is tied to the flywheel, which is tied to the motor, 2000rpm in 6th gear will always be 88mph in my car, whether its going up or down a hill
So you are saying that no matter whether it's a flat surface or a 40% incline you are going to go 88mph at 2000rpm??

CC on my car certainly adjusts rpm depending on whether I am going up or downhill.
If you stay in the same gear and assuming there is no slippage of the transmission then yes.

Your speed is going up or down with the rpm adjustments you CC is making.
 

0roo0roo

No Lifer
Sep 21, 2002
64,862
83
91
Originally posted by: Vic
Originally posted by: psteng19
Originally posted by: Pacfanweb
However, on a fuel injected vehicle, this isn't necessarily the case. Most cars nowadays, if you coast for a few seconds, will completely shut off the fuel. So they aren't burning any gas, which is obviously less than the minute amount that it burns at idle.
OK, so if you coast in gear and the computer shuts off the injectors, what keeps the engine turning?
The wheels (via momentum)?
Do the sparks fire?
The kinetic energy of the vehicle moving forward, transferred to the engine through the transmission from the wheels rolling on the ground.
yea. i don't think the sparks would fire if the cars smart enough right?

anyways its safer to stay in gear. coasting in neutral isn't worth it, regardless of if it uses more or less fuel. the numbers would be insignificant either way
 

thomsbrain

Lifer
Dec 4, 2001
18,148
1
0
Originally posted by: HardcoreRobot
cruise control when i have used it kept my speed constant, not the rpm. meaning, going up a hill, its gonna increase rpm to maintain speed
with any auto tranny that doesn't suck, the torque convertor should be locked at cruising speed, so RPM won't change unless speed changes. that said, my car will not lock the convertor unless it is at full operating temp. in winter, this can take 40 minutes, and going down long hills will pull the temp down enough to unlock it again. it also unlocks if the computer triggers an error code, or when you give it a lot of gas (between light acceleration and full kick-down).
 

Pacfanweb

Lifer
Jan 2, 2000
13,131
54
91
Originally posted by: 5LiterMustang
Originally posted by: HardcoreRobot
cruise control when i have used it kept my speed constant, not the rpm. meaning, going up a hill, its gonna increase rpm to maintain speed
Uhhh negative...you dont increase rpm to maintain speed...2000rpm downhill is the same speed as 2000rpm up hill assuming you're using the same gear. If you're using cruise control then it will vary the amount of throttle, increasing the injector pulse or increasing the amount of fuel the injectors are firing. The tires are tied to the axle which is tied to the flywheel, which is tied to the motor, 2000rpm in 6th gear will always be 88mph in my car, whether its going up or down a hill
If he has an automatic, his rpm's will increase going uphill.
With a stick, you're correct.
 

Pacfanweb

Lifer
Jan 2, 2000
13,131
54
91
Originally posted by: MelikK
Originally posted by: 5LiterMustang
Originally posted by: HardcoreRobot
cruise control when i have used it kept my speed constant, not the rpm. meaning, going up a hill, its gonna increase rpm to maintain speed
Uhhh negative...you dont increase rpm to maintain speed...2000rpm downhill is the same speed as 2000rpm up hill assuming you're using the same gear. If you're using cruise control then it will vary the amount of throttle, increasing the injector pulse or increasing the amount of fuel the injectors are firing. The tires are tied to the axle which is tied to the flywheel, which is tied to the motor, 2000rpm in 6th gear will always be 88mph in my car, whether its going up or down a hill
So you are saying that no matter whether it's a flat surface or a 40% incline you are going to go 88mph at 2000rpm??

CC on my car certainly adjusts rpm depending on whether I am going up or downhill.
If you have a manual transmission, then your rpm does not change at a certain speed in whatever gear you're in.
If you can go 75mph at 2500 rpm in 5th gear, it will always be so. No exceptions, unless your clutch is slipping.
2500 rpm=75mph, period.
If your car adjusts rpm going up and down hill with CC on, then you have an automatic, and it is downshifting. That can't happen with a stick.
 

Pacfanweb

Lifer
Jan 2, 2000
13,131
54
91
Originally posted by: thomsbrain
Originally posted by: HardcoreRobot
cruise control when i have used it kept my speed constant, not the rpm. meaning, going up a hill, its gonna increase rpm to maintain speed
with any auto tranny that doesn't suck, the torque convertor should be locked at cruising speed, so RPM won't change unless speed changes. that said, my car will not lock the convertor unless it is at full operating temp. in winter, this can take 40 minutes, and going down long hills will pull the temp down enough to unlock it again. it also unlocks if the computer triggers an error code, or when you give it a lot of gas (between light acceleration and full kick-down).
So in theory you're right, but in reality you're not. No automatic car will or can remain in the same gear with the converter locked when you go up any significant hill, therefore, you will always have a downshift.
Maybe on a slight incline it'll keep the converter locked, but in high gear, with a locked converter, the rpm's are so low that you can't give it enough gas to maintain your speed up a big hill without downshifting.

 

TheNinja

Lifer
Jan 22, 2003
12,207
1
0
I don't think the difference between coasting in gear and coasting in nuetral is going to cause 8 MPG difference do you? Perhaps you shouldn't floor it at the stop signs? Although my guess is that if you are getting 43 MPG you probably drive like you should be retired and living in Florida.
 

cavemanmoron

Lifer
Mar 13, 2001
13,612
19
81
you get 38 mpg and whine//?

WAAAHH

wtf buy gasoline,and support the economy like I do.

My 1964 Valiant gets about 20 mpg,with the V8 in it.

My 1995 G20 Chevy van gets about 15 mpg.going back forth to work,
do you hear me whine?

Oh yeah if I bought a new $20,000 vehicle to get better gas mileage,if I travel 10,000 miles a year,
how many Years will it take me to pay for the new car,thay gets 30 mpg,vs 15?

at $2.00 per gal. {US Gallon}

Now thats a Real world math problem for ya!! ;)
 

psteng19

Diamond Member
Dec 9, 2000
5,953
0
0
Originally posted by: TheNinja
I don't think the difference between coasting in gear and coasting in nuetral is going to cause 8 MPG difference do you? Perhaps you shouldn't floor it at the stop signs? Although my guess is that if you are getting 43 MPG you probably drive like you should be retired and living in Florida.
No, that wasn't a reference point or anything.
I usually get ~40-44 mpg in the summer, 34-37 mpg in the winter due to differences in the gas, driving style remaining constant.
 
Aug 27, 2002
10,043
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I wish I got 35mpg :( I'm hitting around 16-17mpg now.

edit: the real question is how often to you have to fill up, as in how much do you spend on fuel a week?

I usually spend from $25-$35 a day 5-6 days a week.
 

jagec

Lifer
Apr 30, 2004
24,442
4
0
Originally posted by: cavemanmoron
Oh yeah if I bought a new $20,000 vehicle to get better gas mileage,if I travel 10,000 miles a year,
how many Years will it take me to pay for the new car,thay gets 30 mpg,vs 15?
it's not about saving money. If you want an efficient car to save money, you buy an old Civic. If you buy something like a hybrid, you're buying it for the principle, not for economic reasons.

My Golf COMPLETELY shuts down the fuel when I take my foot off the gas...it jerks it a bit in lower gears, and I actually asked the dealership about it. They said it was normal, and that newer Golfs smoothed the transition a bit.
 

Zenmervolt

Elite member
Oct 22, 2000
24,510
11
81
Originally posted by: conjur
Sounds like a question for Click 'n Clack
They're idiots. They recommended replacing the radiator and heater core on an early 1980's 911 to cure film on the windshield.

That vintage 911 doesn't have a radiator, or a heater core.

Cruise-control does save a little bit of gasoline on flat roads though. It maintains a much more constant speed than a person can do on his own, and all that little speeding up and slowing down wastes gasoline.

ZV
 

EULA

Senior member
Aug 13, 2004
940
0
0
coasting in most states is illegal, and presents an accident waiting to happen.
 

Goosemaster

Lifer
Apr 10, 2001
48,777
3
81
Simple really.

1.Find out how much your vehicle uses at idle.
2. Find out if an when the injectors stop pumping fuel into the engine when engine braking.
3. Do that math and realize that yo uare wasting your time:thumbsup:
 

EULA

Senior member
Aug 13, 2004
940
0
0
a good way to save fuel is to simply shut the engine off when stopped for a while...
 

jagec

Lifer
Apr 30, 2004
24,442
4
0
Originally posted by: EULA
a good way to save fuel is to simply shut the engine off when stopped for a while...
There's a couple drawbridges in Seattle. If they go up, you'll be stuck there for a good 10-15 minutes at least. Do people shut off their engines? NO!:roll:
 

Zenmervolt

Elite member
Oct 22, 2000
24,510
11
81
Originally posted by: jagec
Originally posted by: EULA
a good way to save fuel is to simply shut the engine off when stopped for a while...
There's a couple drawbridges in Seattle. If they go up, you'll be stuck there for a good 10-15 minutes at least. Do people shut off their engines? NO!:roll:
Yeah, 15 minutes at idle. That's almost a whole 1/8 gallon of fuel used! Oh NOES!

Granted, it would make a big difference if everyone did it, but the unfortunate fact is that it's just not enough of an impact at the individual level for most people to bother with it.

For the record, if I'm sitting somewhere and it seems likely that I'll be any more than a couple minutes without moving, I shut the car off.

ZV
 

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