Going Downhill , Use Neutral Or 5Th Gear ?

[omri617 18]

The other day I had a debate with a friend , about driving the 205 GTI when going downhill

Let's put the safety aside (Of course it's not recommended and not safe to drive on Neutral)

When talking ONLY about fuel efficiency , Does the 205 GTI consume less fuel or more than driving on 5th gear when driving in Neutral ?

[JamesE 5]

I'm sure I will be proved wrong (I normally am) but my understanding is that in gear off throttle is the most economical.

[Ryan 99]

Coasting in-gear is more efficient than coasting in neutral. Above a certain engine speed the ECU cuts off the fuel completely when the throttle is closed. Whereas in neutral the engine is idling and using fuel.

Edited June 22, 2013 by Ryan

[omri617 18]

Coasting in-gear is more efficient than coasting in neutral. Above a certain engine speed the ECU cuts off the fuel completely when the throttle is closed. Whereas in neutral the engine is idling and using fuel.

 

If it cuts fuel , how the engine keeps running?

[Slo 283 1 Cars]

I should imagine that in gear or coasting is irrelevant if the throttle is closed it will use same amount of fuel regardless. When you are accelerating and you floor it the car is slow immediately at setting off but it is still using the same amount of fuel regardless of how fast the engine is turning.

[JamesE 5]

 

If it cuts fuel , how the engine keeps running?

It doesn't need fuel to keep spinning in gear

[JamesE 5]

I can't remember which way around but Diesel or petrol - pressing the accelerator adds air to one, fuel to the other

[omri617 18]

I should imagine that in gear or coasting is irrelevant if the throttle is closed it will use same amount of fuel regardless. When you are accelerating and you floor it the car is slow immediately at setting off but it is still using the same amount of fuel regardless of how fast the engine is turning.

 

When coasting in gear , the RPM shows 20-30 (depending on the speed ) how can you know , when doing that , that its using fuel or not?

[Goliath 94]

When coasting in gear , the RPM shows 20-30 (depending on the speed ) how can you know , when doing that , that its using fuel or not?

If you were using fuel you would be accelerating or staying at a constant speed if it's level ground. Coasting in gear uses no fuel if you are a little above idle speed. If you turn the engine off (not recommended while driving) whilst still in gear then you will not notice any change in the engine sound as it will still be turning over without any fuel being injected.

[omega 82]

but if you coast in gear don't you get drag that would use more fuel? also I would think you would go further out of gear than in gear .

[Ryan 99]

Yes that's what engine braking is. There's no fuel, but the engine is still compressing air so it gradually uses up energy and slows down. But going downhill you've got gravity making to go faster, so they cancel out.

[Cameron 16]

I hate it when people put their cars in neutral to coast.. it's basically them silently telling you they know nothing about cars or driving.

 

When you coast in 5th gear your engine braking is minimal due to the gearing, but you use no fuel at all; when you coast in neutral you use fuel to keep the engine idling.

[omri617 18]

How does the car 'know' when to cut fuel off?

Which sensor is telling what?

[Ryan 99]

When the throttle switch is closed and the engine speed above a certain rpm (about 1800 iirc)

[omega 82]

When the throttle switch is closed and the engine speed above a certain rpm (about 1800 iirc)

 

not many cars/205 tick over at 1800

[welshpug 1,657]

He's right, i have an afr gauge on the 205 and you can see when fuel cut kicks in, its around 1800 and only when warm.

[benny205mi16 19 1 Cars]

When rolling in 5th the engine braking although low will still be more resistance than rolling in neutral, so you will therefore need to be back in a drive gear and using fuel again to power the car, where as in neutral the car would cover a greater distance before needing to have power supplied again, so the way I see it is, which uses more fuel here. Do you use more by ticking over in neutral or more to get to where you could of rolled to because you have stopped sooner because of the engine braking? I hope that makes sense, I know what I am trying to say but not sure I have out it into words very well.

[Goliath 94]

When rolling in 5th the engine braking although low will still be more resistance than rolling in neutral, so you will therefore need to be back in a drive gear and using fuel again to power the car, where as in neutral the car would cover a greater distance before needing to have power supplied again, so the way I see it is, which uses more fuel here. Do you use more by ticking over in neutral or more to get to where you could of rolled to because you have stopped sooner because of the engine braking? I hope that makes sense, I know what I am trying to say but not sure I have out it into words very well.

It depends what road you are driving on. If I am driving on a flat motorway at 70mph and want to keep at a constant speed then I will stay in gear with minimal throttle to keep at a constant speed. If I am going down hill and want to keep a constant speed then I will let off the throttle so the engine braking will keep the car back to a constant speed.

I'm not sure why anyone would be quite so worried about fuel consumption in a 205 GTI though! If you get more then 200 miles out of a tank of fuel then you are doing it wrong and need more bwarp!

[AndyCrom 64]

That's a useful gem of info Ryan/Welshpug, so based the fuel cutoff around 1800 you are fully coasting if above.......

 

1.9 gear ratios

3rd - 24mph+

4th - 31mph+

5th - 38mph+

 

1.6 gear ratios

3rd - 22mph+

4th - 28mph+

5th - 34mph+

 

 

On a side note i'm currently getting around 36mpg, gotta love the lack of traffic round here

Edited June 22, 2013 by AndyCrom

[benny205mi16 19 1 Cars]

It depends what road you are driving on. If I am driving on a flat motorway at 70mph and want to keep at a constant speed then I will stay in gear with minimal throttle to keep at a constant speed. If I am going down hill and want to keep a constant speed then I will let off the throttle so the engine braking will keep the car back to a constant speed.

I'm not sure why anyone would be quite so worried about fuel consumption in a 205 GTI though! If you get more then 200 miles out of a tank of fuel then you are doing it wrong and need more bwarp!

I am only talking about going down hill, as the OP stated.

I agree that in 5th will use less fuel at that moment in time because of the fuel cut off, but in the real world because of needing to be back in gear and using fuel again I think rolling in neutral will use less over all.

[AndyCrom 64]

Lets say a car uses 4ltrs or 4000ml of fuel an hour or even 67ml per minute just on tick over while standing still or even coasting in neutral...

 

Next lets say you are driving down a hill (I have one near me thats almost 6 miles long) with constant enough gradient to keep the car above 35mph in 5th gear.

 

Ok so if I drive down the hill at 45mph over 6 miles it would take me 8 mins to cover the distance

 

 

The results would be if I engine brake and use the weight of the car to turn over the engine I can drive for 8 mins covering 6 miles using NO fuel

 

However if i coasted and let the engine run at tickover for 8 mins I would have used 533ml or 1/2 a litre of fuel

Edited June 22, 2013 by AndyCrom

[benny205mi16 19 1 Cars]

Would you not make it to the bottom of the hill quicker than 8 minutes if you roll in neutral, because of the car being able to gain a larger speed than 35mph by having no resistance of the gearbox,

Which I see what you are saying is still using some fuel to tick over, but because of rolling I can now carry on with the speed gained from the hill along the following flat, where as the car in gear is now needing to use fuel again and be driven in gear to get to the same point.

[welshpug 1,657]

No, because you wouldnt want to go that fast, i'd rather be in control of my vehicle also!

 

Theres no flat in mid wales!

[benny205mi16 19 1 Cars]

I understand that in real life you would not just roll as fast as possible, But the OP said 'with safety aside'

So the whole topic is a 'what if' kind of thing, what is possible, Not what would you actually do in life.

[harryskid 193]

Ooo this is sooo interesting but pray what would it be like uphill?