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How far out are the claimed MPG?

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  • How far out are the claimed MPG?

    I have just bought a 2.0 cdti 161bhp eco. The claimed figures are 52.3 urban, 65.7 combined and 76.3 extra urban. Now I am not naive enough to think I would be getting those figures however I am completely shocked at how far away the reality is. Initially I was only getting 42mpg combined. This was measured by putting in £20 as soon as the light came on and then making a note of the miles covered when it came back on. This was repeated twice and I got the same figure. The journey was about 2/3rds Motorway and fast roads and about a 3rd round town in fairly light traffic. I drive very carefully and spend most of the time crawling at 58 on CC.

    Since I have replaced the air filter and disconnected the battery for 3 hours hoping to reset the ecu. I have repeated the above and got a figure of 50mpg. A significant improvement however still a long way short of the claimed MPG and not even as good as my old 2.0dti Vectra.

    The car runs great and there are no faults showing on the dash and if you ask it absolutely flies. Are the quoted figures really that far away from the reality and I am expecting too much or is there something potentially wrong?

  • #2
    The real figures are that far away, and certainly won't account for fuel used during DPF regens etc. Check tyre pressures, and check the fuel rating of the tyres you currently have as this can make a difference.
    Insignia Elite Nav 2ltr SIDI Turbo 250PS, Pearlescent Emerald Green, VX-Line Interior, VX-Line Exterior Styling Kit, Premium Brandy Nappa Sports Leather, 20" Elite Multi Spoke Alloy Wheels, 8" Digital Drivers Display, Bose Premium Audio System, Front Camera Pack, Rear View Camera, Adaptive Cruise Control, Advanced Park Assist, Rear Privacy Glass, Keyless Entry & Start, Tyre Pressure Monitoring System, Factory Fitted Towbar

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    • #3
      https://www.honestjohn.co.uk/realmpg.../insignia-2008

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      • #4
        Thanks for the replies. While nobody wants there to be something wrong with their car those are probably the replies I was dreading. At least if my MPG was not typical there would potentially be room for improvement. How do car manufacturers get away with it? Yes you can allow a few mpg for "ideal parameters" but only out and out lies can account for such a discrepancy. If I would have bought the car new I would be furious. I am so disspointed that part of me wishes I had changed the clutch in my old Vectra.

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        • #5
          Mine's is spot on, claimed consumption is 6.7 l/100km, my car thinks it's using 6.7 l/100km and calculated is also 6.7 l/100km.

          Your way of calculated is rather inaccurate, a better way is to top off the tank, drive a distance and top it off again. There are Vauxhall that calculate remaining fuel based on fuel used.
          The fuel gauge in my Corsa was dropping when petrol level was below 1/4 and started to show the low-petrol light even when it was running on LPG and wasn't using any petrol.

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          • #6
            Perhaps Opel are more honest about their cars consumption than Vauxhall? If you follow the link posted above there is a notable trend. The greater the claimed MPG the higher the discrepancy. Conversely the more modest the claimed MPG the greater the accuracy. It would appear that the worst offenders are the Eco models.

            I concede that as a single exercise my way of measuring fuel consumption is slightly inaccurate. However all the variables are exact except the point when my sender unit triggers the low fuel warning. If all the other parameters are accurate over the same journey the figures will end up as an accurate enough average. This will be confirmed if they tally in consecutive journeys.

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            • #7
              Most of those CO2 reduction stuff is only effective during the test cycles, not in real life. Thinks like an active grill that closes when the engine is cold to reduce drag, will always be open during actual driving. Same as start-stop, the test cycle contains a lot of start-stop traffic. Electric power steering will not consume a lot of power when driving in a straight line as in the test. In real life, is uses a lot more.
              Those things are not present on non-eco versions, making it easier to get close to the claimed MPG.

              My driving is not very typical. I drive a lot of motorway, 220 km a day. About 200 km of that at 100 or 120 km/u without any stopping, my average speed is 93 km/u.
              One improvement in mpg I didn't expect was the switch from a F-label tire to a C-label (NEXEN 2000 to Hankook EVO3), that improved my MPG by about 10%.

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              • #8
                Thank you for the reply Ernst. 10% is a fairly significant improvement. I wonder how much more you would have gained going to a B or even a A rated tyre? Your findings agree with the advice given by 0011728 above. As such I think tyres will be the the area I will concentrate on. My rear ones have little life left in them so, will need to be changed soon irrespective. It probably makes sense to change them all for a more fuel efficient tyre.

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                • #9
                  Same figures to support my claims: Summer period with Hankooks, compared to same period with the NEXEN tyres

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                  • #10
                    How accurate is trip computer? at 60mph (using cruise control and 6th gear) on a flat road mine says 74mpg and 99mpg downhill, even a slight downhill. Uphill it can drop to 17mpg

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                    • #11
                      If mine is anything to go by I would say that the whole computer is wildly optimistic. Both in MPG and range. The only figure that I have found is even remotely close is the average MPG. Even that shows higher than the actual.

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                      • #12
                        Found mine not to be far out. using the brim method. Using the same Tesco pump, fill the tank until the second cut off,drive as normal then repeat the fill up. Worked out 41 mpg combined trip read 43. Not bothered since, just fill tank. When I took my tin tent to Norfolk, for a week, a week or two back. Two hundred miles towing there and back, pitched near Fakenham visited seaside towns around north Norfolk, Put 59 in tank to come home with, then drove as normal at home for nearly the next fortnight, before filling. Not a clue what average was, but was still goodish.
                        Sport Touror Elite 163



                        Dave

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                        • #13
                          Today got 1000 km (621.5 mi) after 3 weeks owning car.
                          It was combined driving with 40% open road and 60% city driving at very high temperatures outside. Up to 40C (104F) with lot of A/C.
                          Refilling tank really surprised me. In a positive way.
                          For 2013 195 HP 4x4 wearing 225/55/17, 56 litres of diesel seemed more like some Škoda or VW (except the fact Škoda doesn't have tank that big ).
                          I really expected over or near 7l/100 (40mpg) and she took exactly 5.6l/100 (just above 50 mpg). Still amazed from the moment I calculated average consumption.

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