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MPG calculations

Discussion in 'Gen II Prius Main Forum' started by Britprius, Sep 11, 2011.

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  1. Britprius

    Britprius Senior Member

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    I have fitted over size tyres to my gen2 Prius. The tyres are Hankook kinergy eco K425 205/55/16. These tyres have a tmp (turns per mile) of 831 against the OEM tyres 851 a difference of 2.4%. Apart from helping to correct the optimistic speedo it will make the odo under read my mileage by 2.4% witch means I believe that when I calculate my fuel consumption I must add this 2.4% to my total miles for each tank. Strangely if I do this my calculated figures come out at virtually the same as the MFD. Am I doing this correctly or am I missing something.
  2. F8L

    F8L Protecting Habitat & AG Lands

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    The speedo is tricky. I am not sure if the odometer is "connected" to it or not. I would worry less about the speedo and check to see that your odometer is reading correctly. Do they have mile markers on the highways in your area or some other way of monitoring/checking that your odometer is correct?
  3. Britprius

    Britprius Senior Member

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    We do not have mile markers, I live out in the countryside can I use the Prius satnav. I would believe the odo will be out because the car will expect 851 turns of the wheels to clock up one mile but the car with my tyres will have travelled about 120ft further.
  4. uart

    uart Senior Member

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    Most people report the MFD calculated MPG to also be a couple of percent optimistic, so that sounds about right.
  5. David Beale

    David Beale Senior Member

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    Use a GPS to check accuracy. The newer ones (SI chip set) are usually accurate to within 10 m or 30 ft (whichever occurs first). ;). Mine often predict accuracy of 5 m if they have an unobstructed view of the sky. The GPS can give you speed (average it out over a few minutes for best accuracy) and distance. Several checks over different roads will remove any doubts on accuracy.

    Remember the car odometer and/or speedometer are NOT "laboratory instruments" and may be out 5% or even more. Also remember the tyres do wear and that means even IF the two gauges were dead on they wouldn't be for long.
  6. Britprius

    Britprius Senior Member

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    Thanks but unfortunately I do not have access to a gps other than the one fitted in the Prius. I am not concerned with the speedo error in the UK they are normally 10% fast this is born out by roadside radar speed signs. The car has to calculate MPG using miles travelled divided by fuel used, and if we calculate the MPG manually we still use the odo to get miles travelled, and the amount of fuel from the pump. This is why I am concerned with the change in tyre size witch if the odo in the Prius is reasonably accurate I can calculate actual distance traveled. If the odo in the Prius is not reasonably accurate non of our MPG figures mean anything. Perhaps some one with standard OEM tyre in near new condition with known TPM (tyrerack) can check how accurate the odo is and post the results.
  7. xs650

    xs650 Senior Member

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    The GPS error drifts slowly so the speed is quite accurate. If you want to check it read your GPS while you are standing still. Mine usually reads 0, sometimes an occasional 0.1 mph. Considering that the satellites are moving at around 18,000 mph and you are moving around 750 mph around the Earth because of the rotation of the earth, calculating your velocity of 0 is difficult as calculating your velocity driving down the highway.

    Curvy roads and rapidly changing car speeds will result in less accurate GPS readings because the typical consumer grade GPS only updates once a second

    Being in a deep canyon, surrounded by tall buildings or very dense trees overhead can reduce accuracy.
  8. GrumpyCabbie

    GrumpyCabbie Senior Member

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    Surely things will change slightly as the tyres wear down. Tyre pressures (under, over or correct) affects things too. It all works out in the wash.

    Stop worrying about it :)
  9. Britprius

    Britprius Senior Member

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    Thanks for your input Grumpy, it does not worry me I know other things affect the readings but the standard tyres blown up as hard as rock with brand new treads would still not have any where near the TPM as the tyres I have fitted even when they get bald. I am just trying to get realistic figures.
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