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Thread: Speed vs fuel consumption

  1. #1
    Join Date
    Jan 1970
    Location
    Central West NSW
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    23,020

    Speed vs fuel consumption

    On my recent trip to Perth and back I have kept track of fuel economy. In addition, I was in a hurry to get there for a conference, which meant that on the trip west, we travelled at a target speed of 110kph where this was legal (probably mostly 105-110), but on the way back there was no such imperative, and except for the second last day (I had ideas of making it home that day, but got too tired), I worked on a target speed of 100kph.

    Having topped up the tank yesterday (so the distance includes an extra trip to town), I now have the figures, which give us an idea of the effects of that extra 10kph. Travelling 3976km to the outskirts of Perth averaged 11.90 l/100km, and on the return trip, 4152km, including a bit of driving in Perth, averaged 10.80 l/100km.

    So it seems that increasing the driving speed by 10% means an increase in consumption of a bit over 10%.

    This is with the Isuzu engine, but since the extra drag is almost entirely from the 'aerodynamics' of the vehicle, I expect the results are applicable directly to any 110 wagon, and probably indicative for any 90/110/Defender. Diesel engines operating in the part of the power band would not have any appreciable difference in efficiency for a 10% change in speed, so I expect the relative results to apply equally to any engine - Isuzu, Tdi, Td5, Tdci.

    Petrol engines, especially older ones, may have efficiency that varies more, which may make the results less applicable, and more streamlined and lower frontal area vehicles may have a smaller effect.
    John

    JDNSW
    1986 110 County 3.9 diesel
    1970 2a 109 2.25 petrol

  2. #2
    Join Date
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    Try a test run at 85-90kph instead of 100-110kph, You will be amazed at the further reductoin in fuel consumption especially in "Boxey" non areodynamic 4WD's also If you are towing a van or a boat with any sort of cab on it the reduction in speed will save you plenty at the bowser and your engine will Love you for it
    You only get one shot at life, Aim well

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  3. #3
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    We did the same test,at those two speeds with the D4.loaded,over 600Km.Actual speed,as per GPS,100,and 110km/hr.

    It used just over 10% more fuel at the higher speed.

    Ambient was 40 degrees,and we had one of those point and shoot thermometers with us.

    Hub and tyre temperatures also increased between 10 to 15 degrees at the higher speed.
    paul

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  4. #4
    Join Date
    Jul 2006
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    When I drove to Perth in May, I had a fairly strong headwind most days across the Nullarbor and used over 12 litres/100km most days.

    On the return trip, I had a few days of tailwinds and averaged less than 11 litres/100km.

    While my ground speed was about the same, my airspeed was probably different by at least 30 km/h.
    Stirling.JPG
    My setup was even less aerodynamic than normal, so wind resistance would have been an even larger issue.

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  5. #5
    Join Date
    Jul 2011
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    Canada
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    Funny but I just did a 500km trip each way. 110 kph on the way up and 100 kph on the way back, total 1000 km. Fuel consumption on the way up was 13.33 L/100 km, the way back was 12.86 L/100 km. Unfortunately this was not with one of my Rovers, it was my 2006 2500HD Duramax.

    In the past I have noticed that my 200tdi RRC can do much better at 90 kph than 110 kph. On one 800 km run I made a point to try and keep it closer to 90 kph and got 7.5 L/100 km, it normally gets around 9 ish on the hwy.

    With my 90 I've only just fitted a 200tdi into it and did a 5 day 900 km combined hwy and off road camping trip. I had a problem with it and could only get 8 psi boost (I've since fixed this and run around 15 psi with much better pickup). With the low boost I got 11.5 L/100 km, I'm expecting this to improve now it is running better.

  6. #6
    Join Date
    Jan 2016
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    iirc drag is not a linear function so 10% difference in speed is not perse 10% difference in fuel consumption but it has been a loooong time

    What I want to point out though (apart from being obvious perhaps) is that in my experience an engine (diesel or petrol) uses a "fixed" amount of fuel at a minimum. The engine has a certain volume and inside that volume an air/fuel mixture needs to be present. When increasing speed the engine turns over more quickly and thus more of the engine capacity get's pumped through from intake to exhaust in the same amount of time. That fact in and of itself will cause you to burn more fuel. So, yes, drag is probably a factor in the increase in fuel consumption, more rpm's will also use more fuel.

    Try driving a large stretch in 4th gear in stead of 5th and see what that does, at the same speed.

    Cheers,
    -P

  7. #7
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    Quote Originally Posted by prelude View Post
    iirc drag is not a linear function so 10% difference in speed is not perse 10% difference in fuel consumption but it has been a loooong time

    What I want to point out though (apart from being obvious perhaps) is that in my experience an engine (diesel or petrol) uses a "fixed" amount of fuel at a minimum. The engine has a certain volume and inside that volume an air/fuel mixture needs to be present. When increasing speed the engine turns over more quickly and thus more of the engine capacity get's pumped through from intake to exhaust in the same amount of time. That fact in and of itself will cause you to burn more fuel. So, yes, drag is probably a factor in the increase in fuel consumption, more rpm's will also use more fuel.

    Try driving a large stretch in 4th gear in stead of 5th and see what that does, at the same speed.

    Cheers,
    -P
    Aerodynamic drag increases as the square of the speed, so fuel consumption increases as the square of the speed (once above the speed where aerodynamic drag represents most of the drag, typically above about 60kph). But this increase is relative to time, and we are interested in fuel consumption per kilometre. Since the time taken is inversely proportional to the speed, the fuel consumption per kilometre is linearly proportional to speed.

    Specific fuel consumption of an engine is the fuel per unit time per unit power, and for diesel engines or indeed for modern petrol engines is fairly constant in the range of powers typically used on the highway, and also fairly constant for different rpm (all within the sort of speeds we are talking about).

    While higher rpm typically involves increased losses from 'pumping' and aerodynamic losses within the engine, since the engine is producing more power, the percentage loss is usually much the same, or even lower - there are fixed losses such as cooling, alternator, power steering pump, aircon, vacuum pump, which represent a higher percentage of power and hence fuel at low power settings. All of these are pretty negligible when talking about 10% differences in speed.

    If you start driving very slowly, where aerodynamic drag becomes insignificant, these become much more important, as most of the other drag forces are proportional to speed not the square of the speed.

    Then there are complications, such as variations in combustion efficiency with engine speed and throttle opening, but it is difficult to generalise about these.
    John

    JDNSW
    1986 110 County 3.9 diesel
    1970 2a 109 2.25 petrol

  8. #8
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    I do like all that guff from JDNSW. Didn't understand a word of it but it was a great read. Keep the right foot up a bit and it'll make a difference whether towing or just driving somewhere, anywhere at any time.
    Gawd! So many getting so uptight and technical about a litre or two. I think I might have to have a drink or two to calm down.
    AlanH.

  9. #9
    Join Date
    Jan 1970
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    Logan
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    20,259
    Road conditions can also make a significant difference.
    Towing our Guardian camper, our Defender used 11.5 l/110 cruising up the Pacific Highway, mostly motorway, from Sydney to Brisbane in the last school holidays.
    Today we did about 600ks from Gayndah to Emerald, much of it climbing and falling, and used 12.5 l/100 Ks. Same vehicle and camper, similar speeds, different road conditions.
    I notice the grey nomads seem to tow their vans at about 90-95 to save fuel.
    2009 Defender 110 2.4. ARB bulbar, Ironman winch, Safari snorkel, Steinbauer chip, AP HD clutch, Lightforce spots, larger tank, Off Road Systems drawer, Traxide 160 controller, Tekonsha brakes, Mulgo seat runners, Uniden UHF, Nuggetstuff seat corners, breathers, Polaris GPS.

  10. #10
    Join Date
    Jul 2008
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    432
    Power required to increase speed is cubed to the speed. So an increase in speed of 10 percent requires over 30 per cent more power. 1.1^3. Dividing distance traveled the fuel by 10 requires 21 per cent. 1.1^2=1.21 or 21%

    Power required is squared again over drag over the same time as the work over that given distance is doubled.

    Or fuel consumption increase at the square of the speed when you divide distance through again when assessing fuel use over a given distance.

    Engines are most efficient at peak torque which is on one measure optimal gas flow. This will effect your comparison as engine efficiency declines either side of peak torque.
    Generally the lowest consumption will be found at peak torque in the highest gear. Of course weight, tail winds, tyre drag, part throttle efficiencies all effect this so it will be difficult to arrive at a perfect figure.
    In my observation most vehicles seem to give their best at 80 to 90 km per hour
    Clive

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