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Coasting in neutral, good idea or bad?
Comments
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It would help if someone with an mpg display could test this theory on a suitably long downhill section. The difference of course would be that if the hill is not sufficiently steep the car in gear is going to slow down significantly and will probaly require some accellerator pedal to keep it going. A car rolling along with only wind resistance is likely to go much further.
So IMHO the 'rolling' car is on the whole likely to save more fuel than the 'in gear' car. And if you feel confident doing it (as I do) then do it.
on an onboard computer when coasting down a hill the MPG readout is -- (ie no fuel being used) when in a gear but with foot off the revs the read is also --
There was an intesting post somewhere on how the onboard computer doesn't know what to do when coasting so displays --
When the computer displays --, the average mpg shoots up. (giving a misleading/false average mpg when coasting)
We know for a fact when coasting in neutral there is engine tickover fuel use.
We know for a fact when rolling in gear the engine shuts off fuel use (on modern cars)
My "friend" has confirmed this by partaking in some neutral coasting where safe and then working out the acutal MPG based on litres of fuel used vs distance covered.
On a tank which includes a lot of neutreal coasting the actual MPG from that tank is lower than you would expect, ie the computer is wrong. By 2-3mpg usually, thus any neutral coasting hasn't helped your MPG as much as you would have hoped.
On a tank which includes a lot of "in gear" foot off the revs rolling the actual MPG matches the computer very closely.0 -
Just out of interest, why would it have to be over 1500rpm (or other Xrpm pre-set by the manufacturer)?
I would have thought the fuel would have cut off if engine speed is lower. Are you saying that if i'm going downhill at say 40mph, it is more fuel efficient for me to have it in 5th gear (circa 1750rpm) rather than 6th gear (circa 1250rpm) ...assuming it is safe for the speed to rise beyond 40mph anyway
I don't know why they chose this figure, presumably it's so if you dip the clutch to change gear, the engine has enough self inertia to keep going when the fuel is reintroduced. Otherwise, if it's too low when you changed gear the engine would just die instantly, and then 'jump' start itself when the clutch was re-engaged. Being both unsafe and unpleasant.
It may be different for each model, but mine is set at 1500, you can feel the engine braking deminish as the revs fall below 1500. I generally aim to have the car in the appropriate gear, so that as the car holds a steady speed, and on the hills around here, 3rd gear will hold the car at 45-50mph (I think around 4000rpm) quite nicely.A friend is someone who understands your past, believes in your future, and accepts you just the way you are.0 -
I don't know why they chose this figure, presumably it's so if you dip the clutch to change gear, the engine has enough self inertia to keep going when the fuel is reintroduced. Otherwise, if it's too low when you changed gear the engine would just die instantly, and then 'jump' start itself when the clutch was re-engaged. Being both unsafe and unpleasant.
It may be different for each model, but mine is set at 1500, you can feel the engine braking deminish as the revs fall below 1500. I generally aim to have the car in the appropriate gear, so that as the car holds a steady speed, and on the hills around here, 3rd gear will hold the car at 45-50mph (I think around 4000rpm) quite nicely.
would it not be better to have the car in 4th and try and achieve an "engine friendly" 2000rpm?0 -
It would be a lot more useful if fuel consumption notation was inverted, so we talked about gallons per mile. In which case an engine on the overrun is using zero gallons per mile. A fuel computer will display this as -- mpg because it's infinite.
However, idling then causes a problem because it would then be infinite gallons per mile.
Time units have to be introduced to overcome that problem.Happy chappy0 -
would it not be better to have the car in 4th and try and achieve an "engine friendly" 2000rpm?
Afraid not, quite simply the car runs away with me, and I end up on the foot brake all the time; beside's my cars done over 130,000 miles being driven like this, so it must like it! I drop the oil every 5k miles.
And the all the hills bottom out to a sharp blind bend, so I find that this is perfect for coming to safe speed to take the bend, without having to touch a thing. Perfect, calm, relaxing safe driving.:cool:A friend is someone who understands your past, believes in your future, and accepts you just the way you are.0 -
:beer: i suppose i should have asked how steep the hill was! LOL0
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No, you don't know that for a fact, My Vauxhall Corsa Manual saysWe know for a fact when coasting in neutral there is engine tickover fuel use.
We know for a fact when rolling in gear the engine shuts off fuel use (on modern cars)
*"overrun" Is a feature where the fuel supply is sutomatically cut offwhen the vehicle is going downhill or under braking.
Where the * indicates that it is an optional item not fitted to all models.
Must have been going down Mt Everest.My "friend" has confirmed this by partaking in some neutral coasting where safe and then working out the acutal MPG based on litres of fuel used vs distance covered.
I would have to witness for myself because the whole area of measuring mpg is a very difficult thing to do accurately.On a tank which includes a lot of neutreal coasting the actual MPG from that tank is lower than you would expect, ie the computer is wrong. By 2-3mpg usually, thus any neutral coasting hasn't helped your MPG as much as you would have hoped.
On a tank which includes a lot of "in gear" foot off the revs rolling the actual MPG matches the computer very closely.0 -
No, you don't know that for a fact, My Vauxhall Corsa Manual says
*"overrun" Is a feature where the fuel supply is sutomatically cut offwhen the vehicle is going downhill or under braking.
Where the * indicates that it is an optional item not fitted to all models.
Must have been going down Mt Everest.
I would have to witness for myself because the whole area of measuring mpg is a very difficult thing to do accurately.
:rotfl: I should have stated modern cars (excluding some models of corsa)0 -
You know for a fact that it is a feature of every other car on the market? I really doubt it. Since what year? i.e were not just talking about new cars, most people don't have a new car.0
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You know for a fact that it is a feature of every other car on the market? I really doubt it. Since what year? i.e were not just talking about new cars, most people don't have a new car.
Probably most of all the post mid 1990's cars on the roads have it. It was fitted to my '86 escort and my '92 Rover, and that was a base model. So I'd guess that all modern cars on the road have it, probably at the point when catalytic converters become mandatory.A friend is someone who understands your past, believes in your future, and accepts you just the way you are.0
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