Fuel consumption - your opinions please.

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wilkobob
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Post by wilkobob »

My zx seems to return somewhere in the region of 40MPG whether theres a trailer on, or not, and doesnt seem to change however you drive it, fast or slow around town.
The only time it changes, it increases, and thats when my girlfriend drives it on a long run (shes got a ligher right foot than me, being a lady n'all) and she gets about 50 - 60 mpg out of it?!
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TomH
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Post by TomH »

edit - wrong topic!
Homer
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Post by Homer »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Robin</i>

Does the 807 have a trip computer and cruise control?
If so try using the trip computer to verify the MPG and if you have cruise control try a longish run from a reset computer and cruise control set to say 60 mph on a motorway or dual carriage way and see what it reads.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
I set it at 55mph, partly so I didn't have to worry about any speed cameras (through some roadworks) and reset the computer. It recorded 35mpg on about the flattest section of motorway we have round these parts. (I would have expected 45+).
Mileometer is bang on according to multimap, and the speedo is pretty accurate according to my dad's GPS.
Lets see what the Pug dealers come up with 'cos if anything it's getting worse. 21mpg after the wife went shopping in it today.[xx(]
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Post by DoubleChevron »

21mpg [:0] Something is definantly wrong.
My CX2500 GTi Turbo isn't a small car, it's 20years old, no doubt lots of wear in most components. Yet it manages 24mpg in heavy traffic and 34mpg on the highway. Yes that is a PETROL TURBO that is 20years old !!
Fuel economy towing a caravan ... I'm not brave enough to check [V]
seeya,
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Post by Seamster15 »

Yep, Somthing very wrong as my Merc ML 3.2 averages atleast 21mpg........
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Post by Homer »

Well I am sure you all want to know what the Pug dealer found?
Nothing wrong they say, that's all you can expect to get they say. The salesman says different so one of them is lying.
On the plus side they didn't charge me anything (under warranty) and it's had a good wash. [:)]
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Post by paranoid »

Took me quite a while to get into diesel driving mode after a petrol, but slowly the Mpg crept as I adapted getting 46 mpg now and I don't really drive like my dad either.[;)]
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Post by Kowalski »

My economy technique is to be in 5th gear as much as possible (from 25mph upwards) and keeping the revs down. I do thrash it when I'm accelarating (just enough to get the soot out from where it isn't needed) but once I'm up to speed 5th.
The best thing you can do for fuel economy is to drive more slowly, I spent several tanks of fuel doing 60mph as my top speed. It was more relaxing not being in a hurry but you really did feel like you were going slowly and I'm not ready for the cap and slippers full time just yet. My 60mph tanks of fuel gave the best economy so far, 53mpg over 4 tanks.
My Dad has Jeep Cherokee TD and the best fuel economy he ever had was 35mpg with his twin axle trailer on and two large stainless steel vesels in it, usually he could only do about 30...
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Post by Homer »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Kowalski</i>

My economy technique is to be in 5th gear as much as possible (from 25mph upwards) and keeping the revs down.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
25mph in 5th in this "car" just isn't possible and going by the instant consumption readout it's mor economical at 30 in 3rd than 4th.
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">I do thrash it when I'm accelarating (just enough to get the soot out from where it isn't needed) but once I'm up to speed 5th.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
It's the technique I've always used for economy. Reach a cruising speed as quickly as possible then stick to it. It's how my dad drives and he gets 55mpg from his Picasso HDi.
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">The best thing you can do for fuel economy is to drive more slowly,<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
It doesn't seem to make much difference, we got the same consumption driving at around 60mph up the congested A1 as we did doing 90mph down the M6.
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"> I spent several tanks of fuel doing 60mph as my top speed. It was more relaxing not being in a hurry but you really did feel like you were going slowly and I'm not ready for the cap and slippers full time just yet. My 60mph tanks of fuel gave the best economy so far, 53mpg over 4 tanks.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
But even at 55mph on cruise control it's only returning 35mpg, which is cancelled out with the time it spends negotiating the local traffic aggravation scheme at 10 to 15mpg (and that's taking it steady).
Well I'm away for a couple of weeks. When I get back it's going to see a specialist reccomended by a collegue. They found a fault with his Golf TDi which the dealer had been trying to fix for over two years. If they agree with the Pug dealer then I'l llive with it untill I can afford to replace it. I figure I may as well go for a V6 petrol (though it will have to be a different vehicle), I probably won't get much worse consumption.
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Post by TomH »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Kowalski</i>

My economy technique is to be in 5th gear as much as possible (from 25mph upwards) and keeping the revs down. I do thrash it when I'm accelarating (just enough to get the soot out from where it isn't needed) but once I'm up to speed 5th.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Economy isn't about revs. It is better to be in the right gear, ie 3rd gear for this speed, at higher revs, because you don't have to press the accelerator pedal so much to maintain your speed.
And as the accelerator is a direct link to fuelling and how much your car drinks then you will use more. I should imagine you have to floor it up hills, and not to mention the strain you are putting on the engine!
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Post by Kowalski »

The fundamental fact about economy is that engines are more efficient at full load than at part load i.e. half throttle is much less economical than full throttle, i.e. if you need a specific power output it is ALWAYS more efficient to produce that power output at lower RPMs.
If you think about it, if you have your car idling and you press the accelarator, the engine goes faster without producing any more useful energy i.e. it takes more fuel to make the engine turn over faster, this is before you give it some work to do like accelarating a car up a hill. The overhead of just making the engine turn over is bigger compared to the useful power coming out of the engine at higher revs. This is why taller gearing gives better fuel econmy. At a higher speed you are passing more gas through the engine and that gas gets heated up, waste energy comes out of your exhaust.
In the past engines could be worn out by labouring them but these days engines are better engineered and it isn't a problem. The AAs current advice on getting good fuel economy is "large throttle openings and high gears".
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Post by Homer »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Kowalski</i>

The fundamental fact about economy is that engines are more efficient at full load than at part load i.e. half throttle is much less economical than full throttle, i.e. <hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Efficiency and economy are not always the same thing.
A vehicle is operating most efficiently at full throttle and maximum torque. It is at its most economical on the overrun.
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">The AAs current advice on getting good fuel economy is "large throttle openings and high gears".<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
The IAM disagree and suggest most people should be using lower gears and moderate throttle for ecomomy and control.
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Post by Kowalski »

You're not suggesting I use 3rd gear instead of 5th on the motorway now are you? [;)] If you understand why 5th is more economical on the motorway, you understand why 5th is more economical at 30mph (or in fact any other speed) too.
Traffic and other hazards that you have to brake for make the difference, but a lot of these can be pre-empted by slowing down earlier, i.e. look ahead for the red traffic lights and slow down early, if you slow down enough they'll be green when you reach them and you don't have to stop.
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Post by fastandfurryous »

What's being missed here is that engines have fuel maps. "full power" is NOT the most efficient way to run an engine. This used to be true on carburettor engines, but is no longer true for Injection ones, and is certainly not true for a Diesel engine.
As it happens, on a diesel engine it is very easy to determine the maximum efficiency (lowest specific fuel consumption) point, as it co-incides with the peak torque figure. IE for a 1.9 XUDT engine, it's 2250rpm. Thus if you can keep the engine as close to this figure as you can, then you have the best specific fuel consumption (almost irrespective of engine load) and hence the best fuel economy possible for whatever speed you want to drive at.
For the average BE3/XUDT combination, this means 3rd for 30mpg, 4th for 35-45ish, and not changing to 5th until around or above 50mph. When I do this for a complete tank of fuel on the 405, I can get way over 50MPG from it.
I agree completely that reading the road, and matching your speed to the prevailing conditions is one of the best ways to improve economy. Stopping and starting again takes lots of energy (and hence fuel)
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Post by TomH »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Kowalski</i>

You're not suggesting I use 3rd gear instead of 5th on the motorway now are you? [;)] If you understand why 5th is more economical on the motorway, you understand why 5th is more economical at 30mph (or in fact any other speed) too.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
I'm no engineer, so I'm not really qualified to comment knowing little about the technical workings of the combustion engine, but I'm just using common sense that dictates to me that you will use a lot more fuel to maintain your speed at 30 in 5th because you will need more throttle action! At motorway speeds or at 30 in 3rd you are at or nearer the efficient level of peak torque, so the car will maintain speed with only light throttle acion...no?
(not to mention better control and responsive-ness of your vehicle which is probably what the IAM are getting at)
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