Fuel efficient driving.

almillar

Excellent article Tom. I'm comfortable with freewheeling when I don't need to slow down so much, and understand the loss of control that occurs from it. Therefore I will do it when I think a set of lights is going to change, but won't do it going down a twisty hill.
Hopefully people won't be so scared to accelerate, and drive a bit more progressively after reading your post. Slowing down, and stopping are the enemy!

Strider590

movilogo wrote: »

On motorway, just follow a lorry.

Preferably not this one....

Hammyman

myhooose wrote: »

When leaving the motorway at a slip road with traffic lights at the end, I sometimes find it better to brake and slow early, taking longer to reach the traffic lights, by which time the lights have changed allowing me not to have to stop and retaining some momentum.

The correct way to do it is to lift off the accelerator at the 200m marker for the exit and use engine braking meaning you use no fuel whatsoever from that 200m marker to the end of the sliproad. You are right about trying to time it so you don't need to stop. That should be the aim for every junction, roundabout, traffic light.

Hammyman

almillar wrote: »

Excellent article Tom. I'm comfortable with freewheeling when I don't need to slow down so much, and understand the loss of control that occurs from it. Therefore I will do it when I think a set of lights is going to change, but won't do it going down a twisty hill.
Hopefully people won't be so scared to accelerate, and drive a bit more progressively after reading your post. Slowing down, and stopping are the enemy!

If you dip the clutch to freewheel then you are wasting fuel as the engine is ticking over. If you keep the clutch engaged and just lift off the throttle, the engine management cuts all fuel to the engine as long as it is above idle speed.

myhooose

Hammyman wrote: »

If you dip the clutch to freewheel then you are wasting fuel as the engine is ticking over. If you keep the clutch engaged and just lift off the throttle, the engine management cuts all fuel to the engine as long as it is above idle speed.

With the clutch dipped you are not turning over the engine so are reducing the rolling resistance of the car and will not loose as much speed. Whether this is enough to offset the fuel used on idle I don't know.

Obviously if you are coming to a full stop it is better not to freewheel as all your kinetic energy needs to be dissipated somehow.

So I guess it really depends if you are trying to trying to slow down a little to time it through some traffic lights or if you are coming to a full stop.

Hammyman

myhooose wrote: »

With the clutch dipped you are not turning over the engine so are reducing the rolling resistance of the car and will not loose as much speed. Whether this is enough to offset the fuel used on idle I don't know.

It isn't. It also means you are not in control of the car.

birkee

Quote:
"when i come up to junctions i press the clutch and freewheel so I dont have to break...weeeee but someone said yuor not in full control of the car."
I think you'll find that 'freewheeling' is an offence.
Quote:
"Well i was under the impression that using gears to slow down used less petrol than braking."
I'd rather replace brake linings than clutch and gearbox.

Older carburreter engines. My memory says that in overun (engine braking) the vacuum in the inlet manifold increases significantly, and sucks in more fuel than driving does.
Engines run most efficiently at a constant speed. Engine design determines what that is. Your car model determines the road speed at that point. (Gear ratios etc.)

That said .... I do freewheel when slowing down, and braking for stopping. Anticipating the stop is essential. Driver skill dependant for safety.
One I've not spotted, is:-
Don't hold your car on the clutch when stopped at lights etc, it's an extra load on the engine, and effects fuel consumption. Plus it's dangerous. What happens if your foot slips off the clutch, or you pick that moment to faint, or someone drives into the back of you.

Inactive

Hammyman wrote: »

That should be the aim for every junction, roundabout, traffic light.

Not if it is a * Stop * signed junction.;)

knightstyle

I remind myself that the quicker you wear your brake pads the more fuel you are using so always brake gently which means thinking ahead and lifting off the accelerator earlier. Some cars like our Auris have can have the immediate fuel consumption readout which certainly makes you conscious of how you are driving, if I was always on overrun I would get 99.99 mpg!!!!

skiddlydiddly

A lot of your points are sound but the bit about acceleration is flawed.If you do a 0-60 for example and do it with steady acceleration, you will use nowhere near the amount of fuel you use doing it by mashing the accelerator.The variable is that as you put your foot down more, a LOT more fuel goes into the engine.Its more complicated than using x amount of fuel at 2000rpm and assuming its double that at 4000rpm because it changes with the amount of throttle you use significantly.
I have done a fair bit of drag racing and over a 1/4 mile I use probably 1/4 litre of fuel as the injectors are working flat out.Now if I slowly accelerated over the same distance I would use virtually nothing as the injectors would barely be pulsing.

I recently had my ECU remapped and asked for an economical zone around 60-70mph for the motorway.I have a wideband lambda fitted(so can accurately monitor A/F ratio)and I am using less fuel at this speed than if I was going slower and can actually see the mixture leaning off, so more revs but less fuel.My motorway mpg jumped from 30 to 38mpg so despite being a boring part of the map, its certainly worth it.