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Car Forum / UK Car Forums / Driving (UK group) / April 2004MPG mystery
Whilst in an Audi A6 last night I noticed that at 0mph, the car's fuel computer indicated 101.0mpg. How does it work that out then? Given such economy, I think that I will start to travel at 0mph more often if I can cover 101 miles using just 1 gallon. Unless I am in a hurry, of course, might otherwise take quite a while... > Whilst in an Audi A6 last night I noticed that at 0mph, the car's fuel > computer indicated 101.0mpg. How does it work that out then? Does it normally show instantaneous MPG or an average? It is probably an average (if only over a metre or two) in which case it might be right. Although you'd think they'd put something in there to fix it when you were stopped! IIRC the BMW I used to drive had an analogue MPG dial that just went off the scale and hid behind some plastic bit when you stopped, that way you couldn't claim it was wrong :-) > Given such economy, I think that I will start to travel at 0mph more > often if I can cover 101 miles using just 1 gallon. Unless I am in a > hurry, of course, might otherwise take quite a while... I can do 120 mph in my car, but that doesn't mean I could do 120 miles in an hour, in fact I think that would be impossible, unless I found a *very* long hill :-) > IIRC the BMW I used to drive had an analogue MPG dial that just went off the > scale and hid behind some plastic bit when you stopped, that way you > couldn't claim it was wrong :-) My Mondeo shows 0mpg when I'm stationary. Which is, IMHO, what all cars should do. I am travelling 0 miles for every gallon of fuel used when stationary :) > > IIRC the BMW I used to drive had an analogue MPG dial that just > > went off the scale and hid behind some plastic bit when you [quoted text clipped - 3 lines] > cars should do. I am travelling 0 miles for every gallon of fuel used > when stationary :) But for the last metre travelled you have used a certain amount of fuel, so the mpg for that particular metre of your car's life is not zero. It all depends on how it works it out, obviously the OP's car is not doing it a very sensible way, IMO it's "nice" to show zero when stopped :-) IIRC the BMW I drove stayed at >90 mpg when stopped. As you lifted the mpg meter went >90 (probably because little or no fuel was being used as you slowed) and then when you eventually stopped it just stayed there (it certainly didn't suddenly spring back to zero the instant you stopped). As you went to accelerate away it came back down to something like 5 or 10 if you hit the pedal ;-) Haven't a clue how it worked it out! > My Mondeo shows 0mpg when I'm stationary. Which is, IMHO, what all cars > should do. I am travelling 0 miles for every gallon of fuel used when > stationary :) Vauxhall got it right in the Carltons and Omegas (and possibly Vectras that have trip computers) - when the speed drops below something like 5mph, the instantaneous mpg reading switches to gallons per hour. Bloody good idea. My A6 displays some sort of random number on the instantaneous mpg reading when stationary, can be 200mpg, can be something low - I think it depends on how quickly you've decelerated, and which particular point in time it's chosen to store the number from. A bit of a dodgy design, but doesn't affect my enjoyment of the car all that much though. Peter > > My Mondeo shows 0mpg when I'm stationary. Which is, IMHO, what all cars > > should do. I am travelling 0 miles for every gallon of fuel used when [quoted text clipped - 9 lines] > chosen to store the number from. A bit of a dodgy design, but doesn't > affect my enjoyment of the car all that much though. A friend has retro-fitted the trip computer from a rover SD1 into is landrover. It must use 16 bit maths, because it shows 65535mpg at over-run! Pete.  SignatureNOTE! Email address is spamtrapped. Any email will be bounced to you Remove the news and underscore from my address to reply by mail > > My Mondeo shows 0mpg when I'm stationary. Which is, IMHO, what all > > cars should do. I am travelling 0 miles for every gallon of fuel [quoted text clipped - 11 lines] > of a dodgy design, but doesn't affect my enjoyment of the car all > that much though. Exactly, if you are worried about fuel economy you want to know the average mpg (eg for a tank of fuel or a particular journey), the instantaneous mpg is not really very useful. You'll probably find your "instantaneous" reading is actually an average over the last few seconds or metres, and the system gets a bit confused when you actually stop. A bit silly they didn't think of that though! > You'll probably find your "instantaneous" >reading is actually an average over the last few seconds or metres, I guess they don't use calculus... Related question - how do milometers work? If you put bigger wheels on, does it f.ck them up? SignaturePaul My Lake District walking site (updated 29th September 2003): http://paulrooney.netfirms.com > Related question - how do milometers work? If you put bigger wheels > on, does it f.ck them up? Yes and no. Bigger wheels are generally accompanied by lower profile tyres, and often don't change the overal rolling circumference all that much, certainly not in percentage terms. So yes, it can affect it, but if you do the sums you'll find out it's a lot less than you'd think. Peter > > You'll probably find your "instantaneous" > >reading is actually an average over the last few seconds or metres, [quoted text clipped - 3 lines] > Related question - how do milometers work? If you put bigger wheels > on, does it f.ck them up? Fitting larger wheels will change the calibration of the milometer but as you normally drop the side wall of the tyre the difference in normally around 1% so most people don't worry about it. (On the other hand if you are fitting 19" alloys to your nova that came with 13" wheels then you aren't going to be able to sort that out with low profile rubber. > Related question - how do milometers work? If you put bigger wheels > on, does it f.ck them up? Yes. SignatureMarc. Please note the above address is a spam trap, use marcc to reply Printing for clubs of all types http://www.jaceeprint.demon.co.uk Stickers, banners & clothing, for clubs,teams, magazines and dealers. > > My Mondeo shows 0mpg when I'm stationary. Which is, IMHO, what all cars > > should do. I am travelling 0 miles for every gallon of fuel used when [quoted text clipped - 3 lines] > have trip computers) - when the speed drops below something like 5mph, the > instantaneous mpg reading switches to gallons per hour. Bloody good idea. Mine will do that, but only shows it in litres per hour.. Just shows what this group is coming to really doesnt it? Someone accidentally posts the same message a couple of times due to the piss poor service offered by Tiscali and then gets slated, despite him appologising as soon as he knew! Even worse, it wasnt spam, viagra or "free" porn - a genuine posting from a genuine motorist in the UK. And you, "MrBitsy" and "Adrian" can find nothing more relevant to say about the subject?? Do you not have a car with MPG results? That I believe would make you lurkers. I think that "cock" is therefore totally appropriate in response to "Mr Bitsy" and there was no need for the "cretin" Adrian to join in on the same topic with nothing of interest to say. To ensure I do not make myself a complete cretin cock penis: A Vectra SRI 2.2 does on average 30mpg overall but, when the vehicle is not moving the MPG readout changes to gallons per hour - usually 0.3 gph. Look forward to reading some more comments related to motoring and not newsgroup etiquete in the future. Spike > Someone accidentally posts the same message a couple of times > due to the piss poor service offered by Tiscali and then gets > slated, despite him appologising as soon as he knew! It wasn't the multiposting, it was the top-posting. It wasn't even just the top-posting, it was the rabid defence of it. > And you, "MrBitsy" and "Adrian" can find nothing more relevant to say > about the subject?? I can't speak for Ray, but I certainly could answer if I found the person asking worthy of a reply. I don't. > Do you not have a car with MPG results? I do, as it happens. > That I believe would make you lurkers. Erm, no. It makes us regular posters. Unlike the OP and yourself. You are lurkers. You've never - according to the self-same Google you post through - posted here before, whereas the OP is just in double figures. How is trying to enforce posting standards "lurking"? It's more akin to trying to police the rule of the road. The OP thinks he's k3wl enough to post wherever he damn well wants, and will wave his willy at people who try to suggest that following the same rules as everybody else might be a good idea. He then went on to state that doing so meant he didn't need to trim the posts he was replying to, to try to maintain some continuity and sense. That's really going to work well across a very long thread, isn't it? Excuse me for being unimpressed. > I think that "cock" is therefore totally appropriate in response > to "Mr Bitsy" Actually, "cock" was in reply to me. Get your facts straight. "Adrian" <spamtrap@achapman.freeisp.co.uk> wrote in a crap message > I can't speak for Ray, but I certainly could answer if I found the person > asking worthy of a reply. I don't. Should you wish to review the "top posting" you complain so bitterly about you would know the original query read: "Whilst in an Audi A6 last night I noticed that at 0mph, the car's fuel computer indicated 101.0mpg. How does it work that out then?" So the question, my friend, is "How does it work that out then?" Not really that hard to understand being the post was only two paragraphs long in the first place. Perhaps you can explain this? How does it work it out? Why 101 mpg? If not, if you cannot answer, shut up and let others with the intelligence to answer the question properly post their replies without having to "trim" your crap first. Mike > A Vectra SRI 2.2 does on average 30mpg overall but, when the vehicle > is not moving the MPG readout changes to gallons per hour - usually > 0.3 gph. So, does that mean that your car when idling uses as much fuel as it would do travelling at at an average of 10mph? I cant believe that can be true James <jdsw_10@hotmail.com> wrote on 24 Apr 2004 15:19:04 -0700: >> A Vectra SRI 2.2 does on average 30mpg overall but, when the vehicle >> is not moving the MPG readout changes to gallons per hour - usually [quoted text clipped - 4 lines] > > I cant believe that can be true Why not? And what gear is that based on? (You can't usually do 10mph in 5th) SignatureDavid Taylor davidt-news@yadt.co.uk "The future just ain't what it used to be." > James <jdsw_10@hotmail.com> wrote on 24 Apr 2004 15:19:04 -0700: > > > [quoted text clipped - 9 lines] > > Why not? 1st law of thermodynamics? Take the car doing 10mph and lift it up with a crane (allowing the wheels to spin) while maintaining exactly the same fuel rate to the engine, what do you think would happen to the driven wheels? scott <spam@spam.com> wrote on Sun, 25 Apr 2004 20:16:23 +0100: >> James <jdsw_10@hotmail.com> wrote on 24 Apr 2004 15:19:04 -0700: >> > > [quoted text clipped - 11 lines] > > 1st law of thermodynamics? Well, my car can go downhill at 10mph with the engine off, using no fuel. It can travel at 0mph whilst idling, using some fuel, or travel at 0mph whilst revving hard, using lots of fuel. It can even travel at 10mph (or more) whilst idling (albeit in gear). I fail to see how that breaks any laws of thermodynamics. > Take the car doing 10mph and lift it up with a crane (allowing the wheels to > spin) while maintaining exactly the same fuel rate to the engine, what do > you think would happen to the driven wheels? They'd speed up. However, it's not quite that simple, since a car that is idling is generally not in gear, whilst one that is travelling at 10mph (under its own power) is in a gear. Having thought about it a bit more, you're probably right that a car that's idling in neutral would use less fuel than one driving at a steady 10mph on a flat road, under its own power. SignatureDavid Taylor davidt-news@yadt.co.uk "The future just ain't what it used to be." > scott <spam@spam.com> wrote on Sun, 25 Apr 2004 20:16:23 +0100: > > > James <jdsw_10@hotmail.com> wrote on 24 Apr 2004 15:19:04 -0700: [quoted text clipped - 20 lines] > > I fail to see how that breaks any laws of thermodynamics. Sorry, I assumed level ground etc etc and not using gravity or wind to help :-) > > Take the car doing 10mph and lift it up with a crane (allowing the > > wheels to spin) while maintaining exactly the same fuel rate to the > > engine, what do you think would happen to the driven wheels? > > They'd speed up. Exactly. So the engine rpm must have gone up too. Then dip the clutch (while maintaining the same fuel supply rate), the engine rpm will probably go up a little more. So now you have a kind of idling but the engine is revving much higher than it needs to, if you then *reduce the fuel supply* a bit the rpm will drop to a normal level of 900rpm or whatever you require. > However, it's not quite that simple, since a car that is idling is > generally not in gear, whilst one that is travelling at 10mph (under > its own power) is in a gear. Indeed, but it is quite simple, all the losses in the gearbox, axle, wheel bearings, tyre/road interface (and a tiny bit of drag) need more power to overcome them, which isn't required when idling. > Having thought about it a bit more, you're probably right that a car > that's idling in neutral would use less fuel than one driving at a > steady 10mph on a flat road, under its own power. > Exactly. So the engine rpm must have gone up too. Then dip the clutch > (while maintaining the same fuel supply rate), the engine rpm will probably > go up a little more. So now you have a kind of idling but the engine is > revving much higher than it needs to, if you then *reduce the fuel supply* a > bit the rpm will drop to a normal level of 900rpm or whatever you require. Nope, when you are 'idling' in gear, the revs are lower than when you are idling at rest. If you switch the heated windows and headlights on at rest, the revs will drop. A modern ECU will increae the fuel, but usually not enough to reach 900rpm. >> Exactly. So the engine rpm must have gone up too. Then dip the >> clutch (while maintaining the same fuel supply rate), the engine rpm [quoted text clipped - 5 lines] > Nope, when you are 'idling' in gear, the revs are lower than > when you are idling at rest. I doubt you could make your car "idle" in gear much lower than the normal idle speed, I might be wrong though. In my car, it idles at about 800-900 rpm IIRC, and if I try and force it to go slower than this in gear things get very shaky ;-) Anyway, the rpm is determined by the gear ratio and the speed and doesn't really have much to do with how much fuel is being used, the ECU can control the amount of fuel independantly of engine rpm. More fuel is required to stop the engine stalling AND keep the car moving than is for just stopping the engine stalling. > If you switch the heated windows > and headlights on at rest, the revs will drop. A modern ECU > will increae the fuel, but usually not enough to reach 900rpm. Surely switching the heated windows and lights on while moving will also reduce the rpm (given constant amount of fuel being used) ??? > > Nope, when you are 'idling' in gear, the revs are lower than > > when you are idling at rest. [quoted text clipped - 3 lines] > rpm IIRC, and if I try and force it to go slower than this in gear things > get very shaky ;-) It isn't much lwer, bu tit is lower. > Anyway, the rpm is determined by the gear ratio and the speed and doesn't > really have much to do with how much fuel is being used, the ECU can control > the amount of fuel independantly of engine rpm. Yes. >More fuel is required to > stop the engine stalling AND keep the car moving than is for just stopping > the engine stalling. Well, on overrun, clearly less fuel is needed. A car which was set to idle at just enough to stop the engine stalling would probably not be acceptable to the driving public. > Surely switching the heated windows and lights on while moving will also > reduce the rpm (given constant amount of fuel being used) ??? Yes, although it is not so noticeable. >> More fuel is required to >> stop the engine stalling AND keep the car moving than is for just >> stopping the engine stalling. > > Well, on overrun, clearly less fuel is needed. We were talking about a constant 10mph, and I thought it would be sensible to consider a situation where the 10mph was not achieved by gravity or a following wind or fred flintstone or whatever :-) > A car which > was set to idle at just enough to stop the engine stalling [quoted text clipped - 4 lines] > > Yes, although it is not so noticeable. And why is it not so noticeable? > >> Surely switching the heated windows and lights on while moving will > >> also reduce the rpm (given constant amount of fuel being used) ??? > > > > Yes, although it is not so noticeable. > > And why is it not so noticeable? Because there is a massive flywheel connected to the engine. FWIW I just parked my car, as I often do, steering the last 30 yards or so in gear but with my foot off the accelerator. That gives about 600-700 rpm so about 10mph in third. Revs pop up to 1000rpm on clutch down, then settle back to about 850rpm, so I would guess at 15% extra fuel per rev, but 15% lower rpm whilst crawling. >>>> Surely switching the heated windows and lights on while moving will >>>> also reduce the rpm (given constant amount of fuel being used) ??? [quoted text clipped - 4 lines] > > Because there is a massive flywheel connected to the engine. Is the flywheel not still connected when you're out of gear? Why is it less noticeable *when moving*? Hint: the heated windscreen and lights will use (more or less) the same power, whether you're moving or not. > FWIW I just parked my car, as I often do, steering the last > 30 yards or so in gear but with my foot off the accelerator. > That gives about 600-700 rpm so about 10mph in third. > > Revs pop up to 1000rpm on clutch down, Why do you think this happens then? What does the ECU do to cause the revs to drop from 1000 down to 850? > >>>> Surely switching the heated windows and lights on while moving will > >>>> also reduce the rpm (given constant amount of fuel being used) ??? [quoted text clipped - 8 lines] > noticeable *when moving*? Hint: the heated windscreen and lights will use > (more or less) the same power, whether you're moving or not. A flywheel is. A much more massive flywheel is connected when the vehicle is moving and in gear. > > FWIW I just parked my car, as I often do, steering the last > > 30 yards or so in gear but with my foot off the accelerator. [quoted text clipped - 4 lines] > Why do you think this happens then? What does the ECU do to cause the revs > to drop from 1000 down to 850? Reduced fueling, by about 15% per rev, as I wrote just after. > > > > > > Surely switching the heated windows and lights on while > > > > > > moving will also reduce the rpm (given constant amount of [quoted text clipped - 13 lines] > A flywheel is. A much more massive flywheel is connected > when the vehicle is moving and in gear. Spot on, and as you point out further down, it requires an extra 15% fuel to keep that "big flywheel" turning compared to the "little flywheel". > > > FWIW I just parked my car, as I often do, steering the last > > > 30 yards or so in gear but with my foot off the accelerator. [quoted text clipped - 6 lines] > > Reduced fueling, by about 15% per rev, as I wrote just after. Sorry I misunderstood what you meant. Glad we agree now :-) > > A Vectra SRI 2.2 does on average 30mpg overall but, when the vehicle > > is not moving the MPG readout changes to gallons per hour - usually > > 0.3 gph. > > So, does that mean that your car when idling uses as much fuel as it > would do travelling at at an average of 10mph? Nope - why do you say it would? cheers, clive >>> A Vectra SRI 2.2 does on average 30mpg overall but, when the vehicle >>> is not moving the MPG readout changes to gallons per hour - usually [quoted text clipped - 4 lines] > > Nope - why do you say it would? Because he thought that going at 10mph would give you 30mpg, probably incorrectly, but I might be wrong(tm). |
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