Fuel Efficient Driving

Whatever you do don't travel at 40mph everywhere!! going slow isn't necessarily the most economical. Also if you are one that does 40mph because you think "it's good for the environment" PLEASE slow down at least when the limit goes to 30! or 20.
I have been stuck behind a ****** drive doing 25mph for about 10 miles, I couldn't overtake as she was in front of a huge lorry and two other cars!
Sorry rant over :-)
Andi.
 
Acme said:
Smooth and gradual with the throttle when accelerating, change gear smoothly and at as low of an RPM as you can without laboring the engine, keep an eye on the road ahead and compensate for distance by easing off of the throttle or decelerating in-gear with no throttle, instead of using the brakes.

Don't drive too fast on a run, there will be a sweet spot for fuel economy/speed on a run. In my car for example, it is 60MPH. On an incline which will soon peak and become a descent, don't be afraid to let the car decelerate a little rather than laboring the engine more, you will be able to use the descent to re-gain speed without using extra fuel.

On a steep descent, you may be able to use no fuel at all by keeping it in gear and using no throttle input. On some long but gradual declines, you might be able to achieve the same by putting it in a high gear and giving no throttle input. For example on my way home from work, there are at least two stretches of road of about a mile long, where I can sit at 70MPH with no throttle input, and the engine resistance preventing me from exceeding this speed.

Tracking and alignment will help fuel economy, as will using the correct tyre pressures.

The type of tyres you use will also have an impact.

In certain cars, the quality of the fuel you are burning can also have an impact on fuel economy.

Remember that auxiliary functions like climate control also sap a small amount of engine power/potential, so in extreme hypermiling circumstances, you could even turn it, and other things, off.

Think that covers everything really... :)

I usually get about 34MPG on my commute, but using all of the above tactics, I can get up to 41MPG.
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You forgot to mention wind deflectors and lowering your car by 5mm? ;)
 
Doing some googling it appears the fuel cut off under zero throttle pressure only happens on petrol engines

Apparently diesel don't engine brake in the same way and hats why newer cars / trucks with auto boxes and diesel engines are auto switching to neutral. If Diesel can't switch the fuel "off" then putting it into neutral will use the least presumably

Just trying to find a reputable source for this.
 
dacads said:
You forgot to mention wind deflectors and lowering your car by 5mm? ;)
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If you have the windows open a couple of CM with wind deflectors, there will be less drag compared to not having them. So windows open vs windows open, wind deflectors could actually have an incredibly minute positive effect on fuel economy.

The same goes for lowering the car. A lower volume of air passing underneath the car means less drag, which could contribute to better fuel economy.

You may have been joking, but yeah... ;) :p
 
Acme said:
If you have the windows open a couple of CM with wind deflectors, there will be less drag compared to not having them. So windows open vs windows open, wind deflectors could actually have an incredibly minute positive effect on fuel economy.

The same goes for lowering the car. A lower volume of air passing underneath the car means less drag, which could contribute to better fuel economy.

You may have been joking, but yeah... ;) :p
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No I wasn't, I knew there were reasons of aerodynamics as you described.

;)

As for me I don't exactly have a powerful car, so prefer not to rev the nuts off it (well I do sometimes) as it doesn't feel right :p Also letting you're foot off the accelerator before turns etc is a good way, see so many loons going in too fast at a roundabout then braking heavily and then carrying on like they're in a race.
 
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My IAM/RoSPa/HPC acquaintances are adamant that the less time you are on the throttle, the better mpg wise - so for example, 30-50 would be more economical if you made swift progress ( no more than 80-90% of the power band, varies by vehicle), block change up and let off the gas asap rather than slowly accelerate up to 50 at low revs as you are on the throttle for longer.

This only applies if you want to drive at the limit ofcourse, trundling along at 30 would almost certainly be cheaper providing you're in the correct gear.

Breaking is wasting fuel so if you can space out the time on the throttle and applying them, will see your mpg increase with the additional benefit of mechanical sympathy.

It also depends on engine, my NA petrol Mazda loses noticeable speed the second you lift off the accelerator, which is great for eco breaking and judging when you will come to a stop, but terrible for trying to cruise with no throttle pressure. My mother's TDI however will happily continue on for ages without any pressure which makes for easy mpg.
 
All down to the weight of the parts which create inertia, flywheel, crankshaft etc, tend to be heavier on a diesel.
 
Freefaller said:
Spot on. :)

As for me I short shift before the car 'gets onto the cams' - fortunately having a torquey V6 I can get away with reasonably high gears without labouring the engine.

That said, I didn't buy my car for fuel economy :p
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I can drive in 6th at 30mph, gears are wonderful torque multipliers.

But smaller engines will have less internal friction which is a massive impact on MPG when cruising, but they often need a longer 'overdrive' gear. Torque is useful in the measure of specific torque as it gives an indication of how well the engine is turning fuel into energy.

Freefaller said:
Another reason why petrol > diesel :p
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Diesels don't have pumping losses and the whole engine power is controlled by fuel input.
 
Simon said:
I can drive in 6th at 30mph, gears are wonderful torque multipliers.

But smaller engines will have less internal friction which is a massive impact on MPG when cruising, but they often need a longer 'overdrive' gear. Torque is useful in the measure of specific torque as it gives an indication of how well the engine is turning fuel into energy.



Diesels don't have pumping losses and the whole engine power is controlled by fuel input.
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is that confirming that the modern common rail Diesel's dont cut fuel when you come off the accelerator like happens in petrol engines ?
 
[TW]Fox;29240492 said:
Doesn't this depend on the individual vehicle and its gearing?

My car will coast for some distance in 8th using zero fuel...
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The law of conservation of energy says that this is impossible unless something different is happening (such as auto switching to neutral etc) between the two scenarios.

0 throttle + clutch up/8th gear means your car's kinetic energy is being reduced by air & rolling resistance plus engine/drive chain friction.

0 throttle + clutch down/neutral means your car's kinetic energy is being reduced by just air & rolling resistance. The engine is being kept running solely by additional fuel being used to idle.

The difference in this case might not be very noticeable mind you.
 
This coasting in neutral or in gear is irrelevant unless you coast up to junctions to stop perfectly without using the brakes
 
Simon said:
This coasting in neutral or in gear is irrelevant unless you coast up to junctions to stop perfectly without using the brakes
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Really? I think it's the opposite.

If you begin to coast (in either example) at the same place, then at the point you have to brake to a stop you'd be going slower (again, perhaps not noticeably so) if you left it in gear vs neutral coasting.

This means when in-gear coasting (1) no fuel was burnt during the coast & (2) less energy converted into wasted brake heat (as you were going slower) - both resulting in very slightly better fuel consumption.

Also with the in-gear coasting, If you don't have to brake to a stop (also less likely given you've slowed down more), then you can apply the accelerator gently and begin to accelerate - zero brakes, zero wasted fuel due to idling.

EDIT: Note, I'm not arguing the driving style/etiquette here - it's obviously harder, sometimes impossible, to drive in this way - just the pure physics.
 
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Personally I coast in neutral with the engine off. Brake pressure is monitered so of vac gets low it restarts aswell. If I'm stopping though it's just easier to leave over run/ light pressure to charge the hybrid battery.

Feels wierd in normal cars as braking is pure energy loss.
 
Outcast said:
WRONG. The distance you have travelled is far further by coasting than you would have achieved otherwise.

I am a "SafEd instructor" (Safe and Fuel Efficient Driving). All new trucks for example automatically switch into neutral to Eco cruise whenever the sensors detect declines.

They also do this just before you crest a hill and use kinetic energy to carry you over the top. This of course means our road speed plunges dramatically.
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Depends on the vehicle, coasting down early in gear is far more fuel efficient than braking later as it cuts the fuel supply. On a descent, if speed can be maintained, again being in gear off the gas uses less fuel than coasting at idle in neutral or clutch down.

Good planning and reading the road ahead is absolutely key in eco driving, this is where Simon's point becomes invalid as good planning there is no need to brake or stop on many occasions, roundabouts are a prime example. Coasting in gear early saves significant fuel compared to going full speed until you need to brake like most drivers do.
 
[TW]Fox;29240570 said:
It isn't.

This functionality was introduced as part of the facelift, my car is a pre facelift. My Dad's car does do it but the circumstances in which it does so are quite limited.
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Really? Mine just launches into coasting mode after about a second off the accelerator / brakes. Everytime.

How long it stays in coasting mode is entirely dependent on the circumstances of course. I need a relatively steep incline to keep 60 mph while in coasting mode without any input. There is one just before my town though, and I can lift off at the top of the brae, and pretty much coast the 1 mile or so to the junction into town without touching anything. But usually only if there is nothing behind me, or anything that is, is a reasonable distance, as when the hill stops, it drops down to around 40mph pretty quick, where the road is clearly a 60 mph stretch of road. But it generally maintains that speed up to just before the junction, where it slows down enough to make the turn before applying the accelerator.

Of course, even with all this eco driving (I do generally drive in eco pro mode, in a relatively sensible manner) I only manage 40 mpg average. However, I do a LOT of dual carriageway / motorway driving at ~75 on cruise. I do notice that if I take cruise off, and sit between 75-85, I gain about 4-5 mpg. Cruise control annihilates mpg's. But I'm too lazy to not use it on drives that are usually 1hr+
 
I take it by engine under load you mean either at low revs where it is sluggish at accelerating or if climbing a hill in a high gear?

I had also read stuff about accelerating hard (not sure if they meant with pedal fully or almost fully depressed) to get up to speed quickly and then changing to a higher gear when cruising at constant speed. From reading about it they seemed to be suggesting that overall this was more efficient. Sounds like what Hiijinx mentioned.

Of course this all assumes no obstructions causing you to brake etc as forward planning is better. But I'm specifically wondering about most efficient acceleration and cruising gear.

I'd always thought low revs and slow acceleration was best and when I'm driving sensibly and trying to be fuel efficient I tend to keep revs lows, gear high and throttle pressed gently for acceleration. Now I'm wondering if I was going about that wrong and should have slightly higher revs before gear changing as well as being a bit heavier on the throttle.
 
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