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cause and effects of changing tire Dia?

Discussion in 'Gen 3 Prius Fuel Economy' started by Bigleftyinaz, Aug 24, 2013.

  1. Bigleftyinaz

    Bigleftyinaz Junior Member

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    Currently stock tire is 215/45/17 with these specs
    Michelin Primacy MXM4
    500 AA A 24.7 overall dia. and 844 revs per mile
    Michelin Primacy MXM4
    Looking to replace with 205/50/17
    Continental PureContact with EcoPlus Technology
    700 A A 25.1 overall dia and 828 revs per mile
    Continental PureContact with EcoPlus Technology
    I believe speedo with be off by showing going a touch faster then really are.Will odometer be off as well? Also what I'm hoping for is better LRR and beter fuel mileage.

    Also with tire be slightly taller ride should be smoother.


    Site comparing size difference
     
    #1 Bigleftyinaz, Aug 24, 2013
  2. rdgrimes

    rdgrimes Senior Member

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    Your odo/speedo is supposed to be off by a few percent, showing you faster than you actually are. They are built that way.

    What you are proposing is the opposite, it'll show you being slower than it does now, by about 2%. So you might come out about "right", but more importantly you are changing the final drive ratio. Is that your intent?
     
    #2 rdgrimes, Aug 24, 2013
  3. Bigleftyinaz

    Bigleftyinaz Junior Member

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    My real intent is wanting those tires,but they are unavailable in my size. So, I want to know, what will happen ? Will my mpg go down or up
     
    #3 Bigleftyinaz, Aug 24, 2013
  4. bwilson4web

    bwilson4web BMW i3 and Model 3

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    Your indicated MPG will decrease toward an accurate number.

    Bob Wilson
     
    #4 bwilson4web, Aug 24, 2013
  5. vskid3

    vskid3 Active Member

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    Actually, in my experience, the odometer is usually correct even if the speedo reads high. Also, the speed over OBD is usually correct, the regular speedo is just meant to show a little high.

    If you get the tires, I would use a GPS to find out what the difference is. Tire sizes aren't 100% exact, especially when you add in different tire pressures, weight, etc.
     
    #5 vskid3, Aug 25, 2013
  6. Bigleftyinaz

    Bigleftyinaz Junior Member

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    Are you saying The MPG that the OBC shows will now be closer to the calculated number ?I can live with that. Does that mean MPG in real doesn't change?
     
    #6 Bigleftyinaz, Aug 25, 2013
  7. bwilson4web

    bwilson4web BMW i3 and Model 3

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    There may be a slight improvement in urban mileage. As the diameter increases, the transmission in effect gets an 'over drive' effect. It turns slower meaning less mechanical drag.

    Over the different Prius models, Toyota has been tuning the engine-transmission. So I'll leave it as more accurate indicated MPG, mph, and odometer but testing the over drive effect is challenging.

    I've had eight years of mods on our 2003 Prius with historical data and same make, tires. I've run three sizes of Sumitomo T4s with data records for each. So I was able to track the effect and though small, it appears to be real. But the 2003 Prius transaxle and engine combination have seen substantial improvements in each of the following releases. There can come a point when increasing the final drive ratio hurts mileage.

    Bob Wilson
     
    #7 bwilson4web, Aug 25, 2013
  8. Bigleftyinaz

    Bigleftyinaz Junior Member

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    According to the site,the difference between the two tires is 1.8% larger.Is that close enough to not change much of anything or should i stay away?

    I've heard good things about that Continental pure contact tire,that i wanted to try it especially with the 700 rating ,but my size is not avil and that 205/50 is the closest. I'd also think the larger tire might have a softer feel for a smoother ride,?


     
    #8 Bigleftyinaz, Aug 25, 2013
  9. F8L

    F8L Protecting Habitat & AG Lands

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    The PureContact is also more fuel efficient than the MXM4 so all speedo/odo tricks aside, it will be a more efficient tire and the 205/50/17 will offer better fuel economy.

    The drawback to that size is most tires are XL rated which is good in terms of tire strength and load carrying (safety) but the tire also rides a but rougher.
     
    #9 F8L, Aug 31, 2013
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