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



  • 56mph is used because it is the highest speed one can travel at without incurring wind resistance. I usually push my driving up to 60mph as wind resistence is still not very significant. The difference in fuel consumption between 60mph and 70mph is very significant, though.

    I regularly drive a 140 mile round trip to see my elderly dad and which in my small car would cost about £13 in petrol (pre price rises). I found that by keeping my speed to about 60 mph I saved a couple of quid per trip and the journey only took an extra ten minutes. It is also less stressful as you can tootle along behind the lorries in the slow lane and don't get bugged by the "can't wait to pass you so I will hang on your tail or overtake on the inside on the motorway and then cut in front of you dangerously in your blind spot just as you are about to pull in to the slow lane" drivers.

    On the subject of engine versus brakes wear I was taught to drive smoothly - my driving instructor's keyword was ANTICIPATION. Using your brakes unnecessarily was a mortal sin in his eyes and smooth driving was the aim. Slowing down by releasing the pressure on the accelerator as you approach a junction is a safety measure and as your speed decreases your gearing should match this in the same way that it should match it when accelerating.

    By the way one way to improve the smoothness of your driving is to put a load of empty bottles in a loose box in the boot and you will then get an audible indicator of jerky driving. And an unsecured planted up and recently watered pot of flowering plants will show you what happens when you take corners too sharply too - a real mess in the boot to clear up even if the pot doesn't actually break!:D
  • tomstickland
    tomstickland Posts: 19,538 Forumite
    First Post Combo Breaker
    kaya wrote: »
    you cannnot accuratley judge fuel efficiency by volume guys and gals, sorry, petrol increases and decreases in volume according to its temperature, the only way to accurately judge fuel economy would be to weigh the fuel in the tank before and after the test period, trip computers in cars arent too good either, putting a different set of wheels/tyres on your car can make a huge difference to the reading also as your changing the outer circumference of the wheels
    The "brim the tank" method is the most reliable way of measuring mpg. If you have a tank size of 11 gallons and the brim point is uncertain to, say, +/-1/4 of gallon the you have an uncertainty of 0.25/11 ie: around 2.5%. If you managed 400 miles on the thankthen the result is 36mpg +/-0.9mpg. That's good enough to see if careful driving is giving you a worthwhile result. The power of averaging over a number of readings will give more confidence in the results.

    Since fuel consumption is a function of engine efficiency and the amount of work done then it's fairly simple to see how to cut consumption.
    On the economy front a petrol engine makes best steady state efficiency at somewhere around 2000-3000rpm.
    Regarding work done, acceleration itself is necessary, and hard acceleration is fine. It's unecessary acceleration followed by heavy braking, or erratic acceleratino/braking instead of maintaining a steady speed that's wasteful. Every braking operation is turning momentum into heat. ie: fuel into waste heat.

    Edit: The expansion coefficient for Gasoline is just under 1000ppm/deg C. The volumetric expansion factor is approximately 3 times this which gives 0.3% per deg C. Say there's a 10% change in temperature between the before and after fuel measurements then there'd be a 3% error. Once again, the error is at a level that's tolerable given the expectations of the experiement. The combined uncertainty from temperature and tank brimming would be around
    sqrt(2)*3 = 4% ish.
    Since a 10% change in fuel consumption would be regarded as meaningful, 4% of uncertainty is fine.
    Happy chappy
  • davetrousers
    davetrousers Posts: 5,862 Forumite
    First Post First Anniversary Combo Breaker
    No one seems to have mentioned the email going about suggesting a boycott of BP and Shell, for those who are interested here it is;

    People on this forum are fed up with this being posted every other day!

    Please edit your post to delete this bit. Thanks


  • rygon
    rygon Posts: 748 Forumite
    I cant remember who said to not use the petrol stations air for inflating your tyres as it is unreliable? What do you use instead. Ive seen those Tyre air compressors made by RAC etc which pump up to 300psi...with me only needing 32psi and the gauge going up in 10's then, to me, that is a lot less reliable. Im sure the stations will have to have their pressure gauges calibrated every so often as well for legal reasons so i cant imagine them being too far out. I agree with the heat of the tyres though but my nearest station is only 2miles away and at a constant speed (ie not using brakes much) my tyres dont get that warm in that little distance
    Smile and be happy, things can usually get worse!
  • harryhound
    harryhound Posts: 2,662 Forumite
    feyaz wrote: »
    Lets all do this, decrease traffic flow and increase congestion even more, if you want to get to places slowly then walk or take the bus.

    Guess the speed that gets the most cars down a stretch of road in a given time?

    If a stop and start and slam on the brakes crash test dummy causes a "ripple" the speed falls below this magic number and a traffic jam then develops. Not to mention the number of tailgating drivers who then have near misses or worst "kiss" bumpers, putting a lane out of action for a good 20 minutes.

  • John_3:16
    John_3:16 Posts: 849 Forumite
    DRIVE with an egg on the bottom of your shoe.

    OK use a pretend egg.
    The measure of love is love without measure
  • Interestingly I did the same the other week driving from Somerset to Sheffield on the same roads, all motorway, and then repeating the journey later on but accelerating more carefully, not using cruise control, and checking the mileage manually and with the car's trip computer. I drive a Honda Accord Tourer with a 2.2 diesel engine and on the journey where I drove "normally" I achieved 43 mpg. However on the more "careful" journey I managed 49 mpg. For a big car, especially an estate, that's quite a creditable increase. Driving an average 12000 miles per year that must be a decent saving. The journey only took me ten minutes longer believe it or not and was more stress free. I do the same all the time now, and notice even bigger savings with my normal urban/rural commute when the speed is lower, in the 40mph to 60mph region. Ever now and again however I do drive more "spiritedly" for short journeys, as I believe it is actually good for the car.
  • It also means that the shortest route is seldom the most economical, as it usually involves lots of junctions and minor roads. (Unless you're travelling on a bicycle, of course, where the cruising speed isn't very high, so distance is more important than cruising. ;))

    I've been monitoring my fuel for a week through my journey to work. I've now worked out that a longer journey- going out of my way to the nearest main trunk road- saves me considerable fuel compared to the shorter journey on B/C roads. The best indicator is how few times I HAVE to change gear or decelerate. The journey isn't quite as scenic though!

    I used to drive a Discovery and got the boy racer out of me then. Fortunately, I've kept that mentality in mind and my 2.2 Diesel hasn't been over 2000rpm in weeks!
  • webwiz
    webwiz Posts: 215 Forumite
    First Post First Anniversary Combo Breaker
    My car gives an instant fuel consumption figure. I dont know how accurate it is but it consistently shows that on a level motorway using the cruise control it does just under 1 mpg less for each 1 mph more between 60 mph and 80 mph. It is a Peugeot 406 diesel.
  • It may seem counter-intuitive, but recent tests show that there is a better way to save fuel. A few months ago I read in auto express magazine about Dr Steve Cousins and his theory that the engine works most efficiently when you open the throttle wide open and then change gear at 2,000 rpm. In auto express they tried it out and bettered the government figures by a fair bit - which is all the mor amazing whem you consider that usually nobody can get near to most of the government figures let alone exceed them!

    Dr Steve Cousins is a former Professor at Cranfield University and runs a company called AXON Automotive that is involved in Ecological Automotive design.
    AXON is now going to build eco cars and Steve Cousins stated "Axon aims to be the most fuel efficient car manufacturer in Europe and much of how we will do this comes from using our specialist carbon fibre technology"

    His credentials seem very good indeed. http://www.axonautomotive.com/

    I cannot find the auto express article but a simple Google search found me what I wanted:


    His tips on eco driving are at http://www.axonautomotive.com/ecodriving.html but I have copied it here (if it works!)


    We tried it and it was hard to remember to do it - but it definitely worked!

    I'd be interested to see what results others get.
    To infinity and beyond!
This discussion has been closed.
Meet your Ambassadors


  • All Categories
  • 343.2K Banking & Borrowing
  • 250.1K Reduce Debt & Boost Income
  • 449.7K Spending & Discounts
  • 235.3K Work, Benefits & Business
  • 608K Mortgages, Homes & Bills
  • 173K Life & Family
  • 247.9K Travel & Transport
  • 1.5M Hobbies & Leisure
  • 15.9K Discuss & Feedback
  • 15.1K Coronavirus Support Boards