Objectives of a drive, and fuel economy

[WS]

It got me thinking about how, along the lines of adapting your drive to make progress, you might pursue fuel saving as the aim. Obvious maybe, but there we are.

What do you guys regarding flexibility, especially for economy? Where are your boundaries?

In my opinion making economy your aim while driving is similarly wrong to putting progress in the first place. Both approaches usually result in a similar problem - going too fast for conditions. If you make fuel economy your key goal it usually makes you reluctant to slow down in the places where you should. This is because you subconsciously want to avoid building up speed again, as GJD pointed out.

Similarly to zacdocbrown, I like to think about economy as a byproduct of good driving, i.e. based on foresight, proactive, smooth and avoiding any action or movement which does not serve a clear purpose. Unncecessary activity wastes energy, and energy always eventually comes from fuel. Another way to put it might be - try to replace the “trying to save fuel” thinking with “trying not to waste fuel” mindset.

If you want fuel economy, don't use the brakes

In my opinion, this should be phrased "drive in such way that you do not have to use the brakes". Trying "not to use the brakes" is the mindset which, in my experience, makes people avoid slowing down in the places where they should. Alternatively, if they do slow down, they do it incorrectly, by going down through gears, instead of using the footbrake.

Gear doesn't make a huge amount of difference.

Of course it does. In many cars, while doing, for example, 40mph, one could use 2nd, 3rd, 4th, 5th or 6th gear. If the speed is stable, usually fuel consumption falls around 5-10% when going one gear higher. In other words, in 6th fuel consumption will usually be at least 20% lower than in 2nd.

[trashbat]

In my opinion making economy your aim while driving is similarly wrong to putting progress in the first place. Both approaches usually result in a similar problem - going too fast for conditions. If you make fuel economy your key goal it usually makes you reluctant to slow down in the places where you should.

Agreed, of course, but I've never talked about making it such a priority that it impacts safety.

I don't aim to use the brakes less, unless this is via application of my existing flexibility between braking and easing off in advance (again, about objectives of a drive)

You could really look at what I mean in the narrow context of gear selection for a clear road, but I don't necessarily want to frame the discussion as that, because other people might go further.

To go to the negative extreme, 'hypermiling' involves lots of techniques that are practically and significantly contradictory to good safety-oriented behaviours, and I don't want to embrace any of those.

Gear doesn't make a huge amount of difference.

Of course it does. In many cars, while doing, for example, 40mph, one could use 2nd, 3rd, 4th, 5th or 6th gear. If the speed is stable, usually fuel consumption falls around 5-10% when going one gear higher. In other words, in 6th fuel consumption will usually be at least 20% lower than in 2nd.

This matches my recent experiences.

[Gareth]

Gear doesn't make a huge amount of difference.

Of course it does. In many cars, while doing, for example, 40mph, one could use 2nd, 3rd, 4th, 5th or 6th gear. If the speed is stable, usually fuel consumption falls around 5-10% when going one gear higher. In other words, in 6th fuel consumption will usually be at least 20% lower than in 2nd.

Generally, though the choice of gears will be only one or two apart, so instead of comparing fuel consumption in 2nd with that in 6th it would be more likely to be 4th or 5th versus 6th gear.

While there are fuel consumption differences between close gears you need to ask how much those differences contribute to overall fuel economy compared with, say, reducing the amount of braking through better observation and planning.

I suspect, as does 7db, that the improvements gained by selecting a higher gear when acceleration isn't required are small enough not to be particularly significant, especially when you take into account the two extra gear changes needed to use a non-flexible gear for a period of time.

[WS]

While there are fuel consumption differences between close gears you need to ask how much those differences contribute to overall fuel economy compared with, say, reducing the amount of braking through better observation and planning.

The fact that you choose to use higher gears is not equivalent to saying that poor observation and planning are OK. These two aspects of driving don't exclude one another; on the contrary, they are closely linked - as you know.

I suspect, as does 7db, that the improvements gained by selecting a higher gear when acceleration isn't required are small enough not to be particularly significant, especially when you take into account the two extra gear changes needed to use a non-flexible gear for a period of time.

It depends on what you consider significant and what you value in driving. In my opinion, if I can arrive at my destination equally safely, equally smoothly and in the same time, but use 5% less fuel just thanks to a more fuel-conscious use of gears (which is possible, but of course depends on what you compare it with), this is a sign of better driving. Of course provided that one considers fuel consumption important. If one prefers to use lower gears for fun (like myself, at times), then why not. But if staying in a lower gear does not improve safety, smoothness or progress, and is no fun, then why waste even a tiny amount of fuel?

[WS]

To go to the negative extreme, 'hypermiling' involves lots of techniques that are practically and significantly contradictory to good safety-oriented behaviours

True.

[7db]

Just to be clear -- I like using higher gears: it lets me go faster. But on the issue of efficiency, I just don't think that it makes much difference to fuel economy - particularly when compared with braking or top speed selection.

It's the pedal in the middle that does all the mpg. Obviously crashing takes the edge off ones mileage as well. Best avoided.

I also agree it's the wrong thing to be optimising.

[fungus]

I also agree it's the wrong thing to be optimising.

As stated in the DSAs book Driving the Essential Skills. "Whilst it may be good to save fuel, it must not compromise safety."

[Grahar]

It is definitely planning and observation that makes the most difference to fuel economy i.e. ensuring unnecessary acceleration and braking are avoided. During my weekly drive in and out of West London it’s incredible the amount of fuel drivers waste accelerating hard and then braking harshly into queues, red traffic lights etc. You would think that it would occur to drivers who regularly drive that this is utterly pointless. It’s very telling of human nature (though completely irrational) that the sense of the ‘speed’ this behaviour creates, convinces them that they are saving time by driving in such a manner!

One thing worth experimenting with in ‘eco’ driving is observing the time it takes drivers to stop, look and pull out (assuming it is clear) when at a junction. I find sometimes that even advanced drivers don’t change their use of the throttle to compensate for a queue of traffic leaving a junction ahead of them. The time it takes a driver to stop at a junction to look left/right and make a judgement that it is clear to go can be built into ones throttle usage when following. If there is more than one car, the waiting ‘time frame’ is stretched exponentially because not only has each driver got to do the looking/judgment procedure but there is an inevitable stop/start ‘shuffling’ up to the give way lines.

It is possible (with consideration to traffic behind) to save fuel by reducing throttle input to try and arrive at the junction when you think the queue might have cleared with the least amount of braking. Of course this is always a best guess, as you can’t always anticipate traffic flow on the road the cars ahead are joining, but the more you try it the better you get at it.

I am definitely turning into a ‘eco’ bore but I have to find some way of making my drive into London more enjoyable!

[MGF]

...During my weekly drive in and out of West London it’s incredible the amount of fuel drivers waste accelerating hard and then braking harshly into queues, red traffic lights etc. You would think that it would occur to drivers who regularly drive that this is utterly pointless.

I think part of the reason is to stop anyone taking their intended place in the queue.

[Grahar]

Yes, that's a good observation. A lot of competitive or 'territorial' behaviour takes place.

One of the factors to manage when employing 'eco' techniques is managing other drivers interpretation or reaction to ones driving. Often the early 'lift off' to keep ones own car moving with a queue ahead is that someone dives into the 'buffer' zone one has created to 'absorb' the stopped traffic ahead, therefore causing one to brake and defeating the overall purpose. Also, the person behind often ends often up latched to your back bumper!

[Ancient]

Yes, that's a good observation. A lot of competitive or 'territorial' behaviour takes place.

One of the factors to manage when employing 'eco' techniques is managing other drivers interpretation or reaction to ones driving. Often the early 'lift off' to keep ones own car moving with a queue ahead is that someone dives into the 'buffer' zone one has created to 'absorb' the stopped traffic ahead, therefore causing one to brake and defeating the overall purpose.

I found when I used to drive an open-top car, that applauding this ehaviour very oviously amused other drivers in the queue (and adjacent queues), who often joined in

Also, the person behind often ends often up latched to your back bumper!

Funnily enough my experience is that these are in the minority. Once it becomes clear what I am doing (which may take several minutes ), the driver behind usually imitates me. Of course my wife still continually asks "Why don't you just keep up with the car in front? Someone may barge in front of you!"

[Grahar]

Yes, that's true, thinking about it they do quite often imitate the 'keep rolling' strategy. I suppose I should have said that they often run up to your back bumper initially before working out what you are doing. I'm not convinced this makes any lasting impression though, despite the fact that they must have surely noticed that they've not had to accelerate,brake/ dip the clutch countless times whilst they have been following you!

The general aggression and complete lack of understanding of 'average speed' displayed in London driving demonstrates how people who are no doubt often highly intelligent and rational, can behave in such a monumentally short-sighted and stupid way when behind the wheel of a car!

[WS]

It is definitely planning and observation that makes the most difference to fuel economy (...)
I am definitely turning into a ‘eco’ bore but I have to find some way of making my drive into London more enjoyable!

I very much agree with all you said. Also the final point; eco driving may sometimes become so monotonous that it's good sometimes to, quite irrationally, do something just for fun

[fungus]

I very much agree with all you said. Also the final point; eco driving may sometimes become so monotonous that it's good sometimes to, quite irrationally, do something just for fun

+1