'Petrol efficiency experiment; an increase of 20%' blog discussion

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  • Dusty_Vinyl
    Dusty_Vinyl Posts: 26 Forumite
    llandaff wrote: »
    Good trial, Martin, and no doubt you saved petrol, but how much? The onboard computer in cars is not reliable and especially so when you are measuring each trip separately. A better way but a little bit more work is what I have been using over many years.
    First start with a full tank; this is not difficult as the cut out on most pumps uses the pressure from your tank. Record mileage at start and again when you next fill up. Using the recorded difference in mileage and the amount of petrol needed to refill the tank it is easy to calculate the mpg. Every time I do this it differs from the computer average. Having tried this on many cars over the years I have always found my method reliable and the computer rarely agrees.
    Another check on the onboard computer is to wait until it flags up the warning: only X miles of petrol left. Try driving those X miles to see what happens (you may need to carry a spare can of petrol!) . :eek:

    Sorry I beg to differ simply as this isn't true, all forecourt nozzles cut out when the fuel (or foam) touches the end of the spout, have a look in the end of the trigger next time before you fill... you'll see a small apeture in the end of the spout... 98% of garage forecourts use ZVA nozzles made by elaflex in Hamburg germany (printed on the side of the rubber gaurd) these use a balanced flow valve whereas commercial filling nozzles (like Husky) use a capillary tube valve... but in both instances its the foam travelling up the fillernext which trips the shut off and not the back pressure from filling. the faster the pump the more foam you get, the amount of foam is also influenced by the contours of the fillerneck.

    to completely ensure your tank is full you should fill until the nozzle trips, then fill - trip, fill - trip as much as possible until you cannot squeaze another penny into the tank.

    You'll also find that there is a difference in the diameter of the spout inbetween unleaded petrol and diesel nozzles as the diesel nozzle is designed to not fit in a petrol vehicles filler next and is wider. All these factors influence the filling speed and amount of foam in the filler neck.

    likewise your tank float will vary in immersion depth in the tank as the temperature changes, it will be more submerged on a cold day than on a hotday and you'll find that the typical accuracy of a fuel tank gauge is about +/-4 to 5%.

    the CPU in the car will use a flow meter which is typically accurate to +/- 2% or better and provide the true MPG based on it's infallable calculation.
  • pault123
    pault123 Posts: 1,111
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    steve6375 wrote: »
    Reply to IdioPhreak

    What I am saying is that 'granny' style driving is not the most fuel-efficient way to drive.
    If you drove at 65mph on a motorway and then took your foot completely off the accelerator until you reached 55mph and then accelerated up to 65mph again (assuming a flattish road, not going up a hill and assuming it was safe to do so) you will get better fuel economy than just driving at 60mph all the way because you have 'coasted' for a lot of your journey at well over 100mpg .


    Try it. i.e. try to take your foot completely off the accelerator whenever you can (but keep in top gear) and don't worry if you have to accelerate up to top speed again after a few minutes as you have saved a lot more fuel coasting than you used to accelerate.

    Obviously consider other road users when you do this!

    This does work I can take my average MPG from 40mpg to 60mpg on a long journey driving at 60mph and on all hills taking foot completely off and occasionally "gliding" on the flats.then bringing it upto 60 again. On a quiet motorway in the slow lane doesn't cause any problems for other motorists.

    Its a lot more successful if I put it in neutral as the "glide" goes on a lot longer due to no engine braking - BUT ive heard this wears the fly plate out so until told otherwise refrain from doing it.
  • Idiophreak
    Idiophreak Posts: 12,024
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    bigpat wrote: »
    I haven't read all eight pages of this thread, but does anyone else use neutral rather than potentially wearing out their clutch for "coasting"? I've only been doing this for about six weeks now and already i'm seeing approx 15% extra mileage out of my tank of diesel (2.0L Vectra 04 reg).

    I'd be interested to see how this compares with the "digital driiving" mentioned above - because whilst you're "coasting" in neutral, the engine's doing all the work of keeping itself going - when you're coasting in gear, the wheels turning does some (or, as above, all) of the work for you...But obviously having the gear engaged means you'll slow down quicker and need to reaccelerate more....interesting stuff.
  • I have a 2.2 Astra Coupe andit is very heavy on fuel 20mpg i now drive to work at no more than 45 mph get there at the same time every day and use at least £10 less in petrol a week, try it! just leave that little bit earlier and sit back and enjoy the journey instead of racing to get there, I cant believe how much i've saved!!
  • harryhound
    harryhound Posts: 2,662 Forumite
    I used to work with a group of colleagues and we frequently had to go to Swansea in S.Wales.
    I'm the only one who made it there and back without needing to fill up, mind you I also never got a speeding ticket.
  • pault123
    pault123 Posts: 1,111
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    bigpat wrote: »
    I haven't read all eight pages of this thread, but does anyone else use neutral rather than potentially wearing out their clutch for "coasting"? I've only been doing this for about six weeks now and already i'm seeing approx 15% extra mileage out of my tank of diesel (2.0L Vectra 04 reg).

    Am I reading this wrong but isn't using neutral the same as "coasting"? :o

    Ie when coasting round a corner you put the car in neutral, OR keep your foot on the clutch which is the same as neutral as no clutch wear is taking place is foot is fully on the floor.
  • redrabbit29
    redrabbit29 Posts: 1,074
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    I drive a Nissan Micra (yea, yea I know!).

    I used to drive about 80-85 mph on the motorway. I've done a few t hings recently to improve my petrol consumption and I've gone from doing about 210 miles to the tank, to 270 miles to the tank.

    1) Inflate tyres to about 1-2psi above recommended every couple of weeks, or check they are OK.

    2) Have absolutely nothing in the car - except maps, coat, etc... whatever I need. No junk that I'm just shifting from A-B.

    3) Keep the reves low and listen to the engine. I used to rev right up, e.g. get to 40mph in 3rd gear. Now I keep the revs as low as possible.

    4) stick to 70mph exactly on the motorway or slower. It adds about 4 minutes to my 16 mile journey so it hardly even makes a difference. Plus it's an easier drive as most of the time I just cruise along in the slow lane between the trucks. I've noticed others doing it too.

    5) Read the road ahead and roll to a stop - takes a bit of practice.



    I heard a friend of mine saying you can also help your petrol consumption by setting off in 2nd gear instead of 1st. Anyone know if this is true?
    Amo L'Italia
  • redrabbit29
    redrabbit29 Posts: 1,074
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    :confused:
    pault123 wrote: »
    Am I reading this wrong but isn't using neutral the same as "coasting"? :o

    Ie when coasting round a corner you put the car in neutral, OR keep your foot on the clutch which is the same as neutral as no clutch wear is taking place is foot is fully on the floor.

    I don't think it is the same - someone correct me if I'm wrong.

    Coasting - keep foot on clutch. No torque on engine and no labour for it.

    Neutral - Still has to run the engine. Someone even said that at a traffic light it's better to be in first than neutral as it's less work for the engine.

    So in that sense coasting is better than neutral - plus it's probably safer too.
    Amo L'Italia
  • alastairq
    alastairq Posts: 5,030 Forumite
    taking a look at motoring sites concerned with 'modern' cars, the general concensus appears to be.....if one's engine is IDLING.....then the ECU pumps fuel into the engine...not a lot, but some all the same.....when the engine is on 'over-run'.....ie throttle shut, in gear..then the ECU shuts off the fuel supply....

    Coasting..ie driving with either the gearbox in neutral , or the clutch pedal depressed, deprives the driver of a major method of control.

    Fine, when 'nothing untoward occurs'...which doubtless will be 99% of the time......but, when that 1% situation occurs........???

    What I find amazing is, how modern technology, regarding engine management, has changed how drivers need to drive to achieve best fuel consumption....compared with the days when the carburettor was 'king?'

    I drive a 20 year old Volvo 740...an estate to boot.

    Unfortunately for my motoring wallet,I occasionally really need its internal space...and heavy towing abilities.....so for day-to-day driving, I am 'lumbered'.

    however, the old Volvo has its 'endearing' qualities, despite looking very tatty.

    it has the 'small' 2 litre engine....and with 'care', I AVERAGE 30 mpg's....which must mean it achieves more than this amount,occasionally?

    [The engine sports an old Bosche fuel injection system...the type that has constant pressure [K-Jetronic] and NO ECU's to interfere.]

    It gets an oil change evry 3k miles or so....whether it wants it or not...and gets 'checked' every day or so....tyres, liquids, etc.

    It's 'normal' run is an entirely rural commute....with the occasional 30 or 40 /50 limit....and few junctions.

    BUT,my driving style has probably the greatest impact on fuel costs......I do NOT exceed any statutory speed limit.....they're there for a valid reason, far be it from me to suppose I 'may know better'....and avoid harsh acceleration.....of which the old volvo is entirely capable of. [a personal question of hypocrisy, concerning my job]

    Strange as it may seem, the old volvo estates had incredibly good aerodynamics.......my car taking an incredible distance to slow from 55 to 30!

    Changing to a roughly equivalent diesel car [or 4x4] achieves little if anything in terms of either economy or fuel costs...with diesel locally being around 10-15pence a litre more costly than the 95 octane leadless jungle juice it currently consumes......

    Also, since I do all my own servicing and repairs....and of the latter the volvo has required incredibly few over the past 5 years, those costs......[it appears, usually many hundreds of pounds when done by garages]....are drastically reduced.

    SO...it actually seems quite a cost-effective motor?


    Mind....at the other extreme, a colleaguehas just acquired one of those massive Chrylsler estate cars.....a V6 diesel....and he claims to be averaging 46 mpg's.......which is all a tad sickening??
    No, I don't think all other drivers are idiots......but some are determined to change my mind.......
  • pault123
    pault123 Posts: 1,111
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    alastairq wrote: »


    Mind....at the other extreme, a colleaguehas just acquired one of those massive Chrylsler estate cars.....a V6 diesel....and he claims to be averaging 46 mpg's.......which is all a tad sickening??


    VW, Audi and Skoda Estates are commonly fitted with the superb 1.9 TDI diesel turbo injection engine, some common rail, other pumpe duse (faster and more efficicient) its easy to achieve 50-60mpg :beer:
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