Fuel consumption indicators

Su Campu

I have a (stupid?) question.

I have been doing a bit of experimenting on trying to reduce my fuel consumption, doing things like cutting out needless accelerating, braking, watching ahead, proper tyre pressures, driving easy the first few kms when the engine's cold, etc. Another thing is freewheeling (ie. letting the car drive in neutral under its own momentum). My thinking behind this is that in doing so, the engine RPM is around 700, therefore consuming less than if I had it in gear, where it would be around 2200rpm.

Is this a correct assumption? I ask because I recently drove an Opel Zafira with instantaneous consumption indicator, in litres/100km. At idle it was reading ~1.5ltr/100km, while driving at ~120km/hr in 6th gear gave around 7ltr/100km. But when I freewheeled at that speed, consumption went to 0.0! I take it that can't be correct, as the engine is still ticking over and hence injecting fuel into the cylinders.

robbie_998

is it not when you leave it in gear and take your feet off everything and let the car roll that you use no petrol/diesel at all ?

i thought neutral used fuel ?

Absurdum

Su Campu wrote: »

Another thing is freewheeling (ie. letting the car drive in neutral under its own momentum). My thinking behind this is that in doing so, the engine RPM is around 700, therefore consuming less than if I had it in gear, where it would be around 2200rpm.

You are not in control of the vehicle when doing this, and won't have the decleration of the engine to help in slowing down. Also, if your engine stalled, your brakes will lose the assistance of the servo unit, resulting in a severe loss of braking power.

Not worth it whatsoever.

djimi

If you are freewheeling you are not in control of the car. Bad idea.

Absurdum

Absurdum wrote: »

You are not in control of the vehicle when doing this, and won't have the decleration of the engine to help in slowing down. Also, if your engine stalled, your brakes will lose the assistance of the servo unit, resulting in a severe loss of braking power.

Not worth it whatsoever.

Why would it stall when feewheeling ?

Absurdum

RoverJames wrote: »

Why would it stall when feewheeling ?

Good question :P

I'd be afraid the OP might lose the run of him/herself and turn it off to save another 3ml of fuel!

stevenmu

Most modern engines will stop injecting fuel when you are coasting in gear and your momentum is turning the engine, but they will use fuel to keep the engine idling if you put in the clutch; so usually it is recommended to keep the car in gear.

Anan1

Su Campu wrote: »

Is this a correct assumption? I ask because I recently drove an Opel Zafira with instantaneous consumption indicator, in litres/100km. At idle it was reading ~1.5ltr/100km, while driving at ~120km/hr in 6th gear gave around 7ltr/100km. But when I freewheeled at that speed, consumption went to 0.0! I take it that can't be correct, as the engine is still ticking over and hence injecting fuel into the cylinders.

They're not that accurate, idling in neutral at 120km/h will use very little fuel per km travelled.

stevenmu wrote: »

Most modern engines will stop injecting fuel when you are coasting in gear and your momentum is turning the engine, but they will use fuel to keep the engine idling if you put in the clutch; so usually it is recommended to keep the car in gear.

That's a safer way to do it, but it is less economical as the engine is braking the car.

Bodhan

My car does the same thing, I understand that with the clutch in the car uses enough fuel too keep from stalling. Not sure why the fuel usage goes to zero when you coast with the car in gear

biko

I'd say that freewheeling won't have enough effect on the total consumption to be worth doing.
Stop/start techniques and watching your speed on the motorway will have a bigger impact. I find that with the right speed/driving technique on the motorway I go from 30MPG to 40MPG, which is a massive saving driving from Dublin to Galway.

Su Campu

Su Campu wrote: »

Is this a correct assumption? I ask because I recently drove an Opel Zafira with instantaneous consumption indicator, in litres/100km. At idle it was reading ~1.5ltr/100km, while driving at ~120km/hr in 6th gear gave around 7ltr/100km. But when I freewheeled at that speed, consumption went to 0.0! I take it that can't be correct, as the engine is still ticking over and hence injecting fuel into the cylinders.

Sorry, I wrote the wrong thing in red, I meant when I lifted off the accelerator in gear, it went to 0.0. When I freewheeled, it went to to the idle reading of ~1.5ltr/km.

But this has since been confirmed anyway. Thanks for the replies. I know freewheeling is not the best idea, which is why I was uneasy doing it, and wanted to know if it made any difference at all. But now I know that lifting the foot off the gas is the way to go, plus the other things I've been doing.

GTE

Bodhan wrote: »

My car does the same thing, I understand that with the clutch in the car uses enough fuel too keep from stalling. Not sure why the fuel usage goes to zero when you coast with the car in gear

stevenmu said it really. Its because the car moving turns the wheels and turns the engine. The engine is happy with that as its well above idle speed.

When you take your foot off the pedal you dont keep going at the same speed. You do it to slow down slightly. The engine cuts off the fuel to save it in the instance that you want to slow the car down.

My engine will not inject fuel above 1200 rpm unless I ask for it. Ie. Unless I want to speed up.

If Im in 5th and Im coming up to a junction with no traffic I always take the foot off the pedal and go up to it in 5th. Then if its a roundabout by the time the engine starts to struggle with 5th at such a slow speed then its the perfect speed for 2nd or 3rd for the turn. Works out nicely.

Anans point about it being less economical to do this, I dont understand it.

keithclancy

Service your car on a regular basis,

Did mine last weekend and the fuel consumption at 120km/h went down from 5.8L/100 km to 5.1L/100km

Mainly due to the air filter being clogged.

Anan1

bbk wrote: »

Anans point about it being less economical to do this, I dont understand it.

If you take your foot off the accelerator completely while in gear you're using the engine to brake the car. If the car was in neutral then it wouldn't have the drag of engine compression, so it would continue further before coming to a halt.

keithclancy

Plus your using fuel when coasting rather than using momentum to turn the engine:

http://www.popularmechanics.com/cars/how-to/4230705

A: That depends. The engine isn't braking the car going downhill if the transmission is in Neutral, so economy would seem to be high. But if you think the engine is still using fuel while coasting downhill in gear, you're laboring under a misconception. Most fuel-injected cars turn the fuel delivery completely off when you lift your foot from the accelerator. They still burn fuel when idling in Neutral, so do the math. The amount of fuel burned at idle over, say, a couple of miles of coasting downhill is small, but it's still more than zero. So if you're driving a modern fuel-injected car, you're wrong.

Older, carbureted cars would suck gas through the engine while coasting in gear, even if you turned off the ignition switch. In this case you'd be right.

GTE

Anan1 wrote: »

If you take your foot off the accelerator completely while in gear you're using the engine to brake the car. If the car was in neutral then it wouldn't have the drag of engine compression, so it would continue further before coming to a halt.

Well of course that is correct technically but it's an absolute non point considering you shouldn't let the car go in neutral in that situation in the first place.

I thought it was going to be something along the lines of changing down through the gears ending up costing more then the fuel you save. Something there was a discussion on before and definitely a bigger percentage of people going down through the gears then people coasting.

stevenmu

Anan1 wrote: »

That's a safer way to do it, but it is less economical as the engine is braking the car.

True, I was thinking of when you are coming to a complete stop and braking anyway; in this case it's better to brake in gear to a stop rather than to brake in neutral to a stop.

Anan1

bbk wrote: »

Well of course that is correct technically but it's an absolute non point considering you shouldn't let the car go in neutral in that situation in the first place.

Of course not, but it is more economical.

bbk wrote: »

I thought it was going to be something along the lines of changing down through the gears ending up costing more then the fuel you save. Something there was a discussion on before and definitely a bigger percentage of people going down through the gears then people coasting.

I vaguely remember that too, something about wear & tear I think.

stevenmu

bbk wrote: »

I thought it was going to be something along the lines of changing down through the gears ending up costing more then the fuel you save. Something there was a discussion on before and definitely a bigger percentage of people going down through the gears then people coasting.

Repairing or replacing a clutch or gearbox definitely costs a lot more than changing brake pads anyway.

JHet

In Gear with your foot off the Accelerator, a modern engine uses no fuel.

So, this is useful for coasting toward traffic lights and so on, where you are naturally going to be slowing down......coasting down a hill in gear is also another use.

De-engaging a cars gears by placing the car in neutral is a bad idea while driving.

I actaully believe the "In Gear method" is more efficient in the long run, if you think of instances where you can use it versus the free-wheeling method.

Also keeping the car in a higher gear low rev's can help greatly. You need to balance this with the amount of force you put on the Accelerator. If your pressing down harder because you are in a higher gear, this may be less efficient as more fuel is dumped into the engine unnecessarily so there is a certain balance that needs to be attained between gentle acceleration, gearing, coasting in gear and dipping the clutch just before the suddering point, for whatever gear your in.

Hope this helps.

Pkiernan

To save the most fuel:

1. Drive slower - speed is the biggest killer of fuel consumption.
2. Windows up.
3. AC off.
4. Tyres properly inflated.
5. Service regularly - save money by doing oil & air filter change yourself.