Tire size mileage question for the math experts.

Discussion in 'Gen 2 Prius Main Forum' started by swi66, Mar 4, 2011.

  1. swi66

    swi66 Member

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    About 6 months ago I installed the Michelin Hydro edge tires on my Prius and noticed a significant decrease in mileage. They are of course the larger size because the Hydroedge is not available in the same size as came on the Prius.
    Took a little trip last weekend and used the Garmin Nuvi GPS for directions, the first time I used it.
    I noticed the Garmin indicated my speed was precisely 2 mph slower than indicated on my speedometer.
    Good thing to know!

    Now my question?
    How does that translate to the indicated mpg on the Prius readout?

    I knew I was going to take a hit on the mileage due to the tires, but is my actual mileage more or less than on the readout now?
     
  2. uart

    uart Senior Member

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    Hey swi66. It would help if you posted your new tire size is.
     
  3. tonyrenier

    tonyrenier I grew up, but it's still red!

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    I have noticed a 2.1 mph discrepancy since day one on my Gen III. As far as tires and mpg your additional width and presumed additional weight could cause a fairly large decrease by Prius standards.
     
  4. jdenenberg

    jdenenberg EE Professor

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    Go on Tirerack.com and look up the specifications for your tire and the previous tire. The Revolutions/mile is the one to look at. If your new tire has x% fewer revs/mile, the MPG readings will artificially go down by that %.

    You also have to take into account the wear on your old tires. Tire diameter is approximately 24.5" (Check the spec again). A worn tire has a smaller diameter (by about 0.5 inches) than it had new. The reduced diameter increases revs/mile (and apparent MPG) by the same percentage.

    JeffD
     
  5. qbee42

    qbee42 My other car is a boat

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    This is backward from what you would expect. Larger tires will turn more slowly for a given speed, so your speedometer speed should be lower, not higher, for a given real speed. Did you really state this correctly?

    Generally speedometers read high, so you may have partially corrected the error with bigger tires. Odometers, on the other hand, are generally fairly accurate. To determine the effect of tire diameter on your mileage calculation, you need to check the accuracy of your odometer, not speedometer.

    Tom
     
  6. swi66

    swi66 Member

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    I guess I didn't state this correctly
    Correct in my head, but that doesn't always translate to the screen well.
    My spedometer reads 55, the Garmin says I am doing 53.
    I knew there was going to be a slight difference in speed, but untill using the Garmin, I didn't know which way.
    The tire size I have now is 195x60x15
    I beleive the integritys that came on the car were 185x65x15

    Now there is a lot of things going on with the winter weather.
    And now poor quality of gasoline with 10% ethanol.
    But with the integritys I was getting 50-54mpg in good weather.
    during the winter, never got below 44 on a snowy day. Slightly higher just during cold weather.
    Now with the Hydros, I'm averaging 39-43 during the cold weather.

    I was wondering if part of that is attributed more to the larger size making the mileage incorrect towards the lower side.
    Or can most of it be attributed to the larger footprint of the tires, and the poor gas.
     
  7. uart

    uart Senior Member

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    Believe it or not the 195x60x15 is actually a slightly smaller tire (regarding rolling circumference) than is the standard 185x65x15.

    So this has actually made your true mileage slightly less than your indicated mileage, but only by a small amount.

    BTW. The reason it's smaller is because 60% of 195 is slightly less than 65% of 185. :)
     
    Zeppo Shanski likes this.
  8. fuzzy1

    fuzzy1 Senior Member

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    This doesn't matter. The speedometer and odometer have different errors. The first must read high due to various regulations, the second must not read high due to class action lawsuits about warranty fraud.

    Use the Garmin to check your odometer error. And if it is a hiking unit, instead of an automotive unit, make sure that it is not in battery saver mode, which causes considerable trip distance error.

    New tires commonly cause reduced mpg until they are broken in.
     
  9. David Beale

    David Beale Senior Member

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    There are several issues here, as alluded to above.
    Some are:
    1.Tire revs. per distance may be different.
    2.Larger tires have -wider- tread, which will translate to more friction and poorer mileage.
    3.Rain and winter tires have softer rubber which will translate to more friction and poorer mileage. However, the better traction may inspire the driver to not slow down so much in corners and therefore loose less energy.
    4.Tire weight may be different. More weight will mean more energy is lost accelerating them and more is thrown away decelerating them (even though some is recovered by the regen braking).
    5.A different tire diameter may cause the HSD to operate in a different part of its' efficiency graph. For example, at some speed that you travel at for a long time, one tire may run the engine at, say, average RPM of 2100, while another tire may run it at 2200. The efficiency will be different.

    As you can see, it's not a simple issue to solve. You need to be careful in your calculations and measurements due to the many variables.
     
  10. F8L

    F8L Protecting Habitat & AG Lands

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    This may not be a tire size issue.

    I am tracking MPH on the dash and on my ScangaugeII and the SGII consistantly reads 2mph slower than the dash. I have not tracked MPH using the GPS on my phone and my Garmin broke so I cannot add that data but I have a feeling the vehicle simply has a high reading speedometer.

    I should also state that the MFD instantaneous MPG readings nearly always differ from the SGII's readings. They are always very close but they do this little dance were very rarely do they sync.
     
  11. Lloyd9

    Lloyd9 Junior Member

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    I changed Tires to Michellen HydroEdge 205/55R16s a while ago.
    I did see a slight reduction in reported milage on the MDF but I attributed to the change in tire size. However the actual mpg would remain the same. Overall i am happy with the tires, there are specific higher milage, but I needed rain capable tires as well. Small trade offs.

    http://priuschat.com/forums/gen-ii-...g/77904-205-50r16-07-touring.html#post1087989
     
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