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Too Good to be True Mileage increase?

Discussion in 'Gen 3 Prius Fuel Economy' started by Willem, Jun 25, 2012.

  1. Willem

    Willem Junior Member

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    I will spare you the details, but I had to replace all my tires and wheels on my 2010 Prius "V" package. I was at the tire dealer and didn't have time to do research here at PriusChat or TireRack beforehand, so I had to reply on the salesperson to recommend a tire. All I knew was that I wanted Low Rolling Resistance because I had glanced at forum posts in the past.

    I went from the 17 inch factory stock rims and OEM tires to 16 inch Goodyear Assurance tires.

    I immediately took a 300 mile round trip trip that was 85% flat interstate. I kept the speedometer cruise control on 60-65 most of the time. Air conditioning on. Three people and luggage.

    I watched the trip computer move as high as 55.1 MPG. I got home, and it was 53.9 mpg and average speed of 54 MPH.

    I am astonished because I make this trip all the time and previous trips averaged between 43 to 47 MPG at the best. Up to now, I had NEVER broken 50 MPG average.

    Granted, I did drive steadier and slower than usual. But I have *never* gotten such good mileage.

    So, I have concluded that there must be something to these tires. I wonder how much of the increase is attributable to the size (diameter) of the tires versus the tire design.

    Anyway, I was just so happy about my mileage that I wanted to share the news with you.
     
    #1 Willem, Jun 25, 2012
  2. F8L

    F8L Protecting Habitat & AG Lands

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    So you bought new wheels too?

    What tires and wheels did you purchase and what are you doing with the OEM 17" wheels?
     
    #2 F8L, Jun 25, 2012
  3. Willem

    Willem Junior Member

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    The tires are Goodyear Assurance ComforTread Touring P205/55R 16. The OEM 17" wheels already have been sold.
     
    #3 Willem, Jun 25, 2012
  4. F8L

    F8L Protecting Habitat & AG Lands

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    So compared to a good set of LRR 16" tires like the Energy Saver A/S or Ecopia EP422 I would say you are losing about 2+mpg.
     
    #4 F8L, Jun 25, 2012
  5. ScottN

    ScottN Junior Member

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    I had the Goodyear Assurance tires on my previous Prius, and I liked them a lot. As far as your mileage change, going from a 17" to 16" rim is going to cause your speedometer to be off and I am not sure what else it may impact electronically.
     
    #5 ScottN, Jun 25, 2012
  6. Willem

    Willem Junior Member

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    That's interesting. The sales person said this would not be the case. He said because I was going from a low-profile tire to a more "regular" one, the actual outer diameter would be the same.

    I assume I can take my stand alone Garmin and check the speed, and talk with the Toyota dealer and/or others here at PriusChat to remedy.
     
    #6 Willem, Jun 25, 2012
  7. F8L

    F8L Protecting Habitat & AG Lands

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    Your tire store sold you the correct size as a replacement for the OE 17" tire. Your speedo should be just as off as it was from the factory which is 1-2 mph too fast.
     
    #7 F8L, Jun 25, 2012
  8. Willem

    Willem Junior Member

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    F8L Are you suggesting there is a more appropriate size of tire for my application? I can probably (maybe) return these tires for a different size if you think that is a good idea...
     
    #8 Willem, Jun 25, 2012
  9. xliderider

    xliderider Senior Member

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    No, F8L said they sold you the correct size. Your speedo will be as "accurate" as it previously was, within 1 - 2 mph from actual.

    The other comment made previously was incorrect. As long as the overall diameter of the new tires is close to the OEM diameter, there will be no speedometer inaccuracy regardless of the rim diameter.

    There are numerous tire calculators available, but here is one:

    Tire size calculator

    Plug in the tire sizes and it will tell you that you are within a 1% difference on your speedo using your new tires compared to your OEM tires.
     
    #9 xliderider, Jun 25, 2012
    F8L likes this.
  10. Willem

    Willem Junior Member

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    Thanks for the clarification!
     
    #10 Willem, Jun 25, 2012
  11. F8L

    F8L Protecting Habitat & AG Lands

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    Aye, xliderider is correct. :)

    The tires you chose are the correct size. They may not be the most fuel efficient but they have a very long tire life so they are not a bad choice.
     
    #11 F8L, Jun 25, 2012
  12. Willem

    Willem Junior Member

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    Awesome, dude. I had to make such quick decision, I was afraid I might have done something terribly wrong. :)
     
    #12 Willem, Jun 25, 2012
  13. F8L

    F8L Protecting Habitat & AG Lands

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    Nope, you did well young Jedi. :)

    Now should you decide you want the most economical tire and are willing to give up a tiny bit of comfort then you could switch them but it may not be worth the hassle.
     
    #13 F8L, Jun 25, 2012
  14. spiderman

    spiderman wretched

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    I don't get it; what was Willem's Prius IV doing with 17" OEM rims?
     
    #14 spiderman, Jun 25, 2012
  15. xliderider

    xliderider Senior Member

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    I was confused too, but I think his model IV designation in his forum user profile is incorrect.
     
    #15 xliderider, Jun 25, 2012
  16. retired4999

    retired4999 Prius driver since 2005

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    I had the Goodyear Assurance tires on my previous 2005 Prius for seven years, really liked them. I think you will too!!
     
    #16 retired4999, Jun 28, 2012
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