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How I gained 7 MPG overnight!

Discussion in 'Gen 2 Prius Fuel Economy' started by mike_m, Aug 12, 2006.

  1. mike_m

    mike_m New Member

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    I put the stock tires and wheels I bought from Bjorn (DocVijay) on the car last night. I was really shocked at how much of a difference that could really make. I don't know if I can get used to the look of these, vs. mine. I ordered my car with the 17" combo from the dealer so I never had the stock tires as a MPG baseline. I have been averaging about 39 MPG with this brutal heat. I put the stocks on, and immediately was at 46MPG. My trip was only 20 mins. long, so I'm sure it will do even better if I drove it longer. My fist 5 minutes on the MFD was in the high 30's! I have never seen that before. I have wanted to try this for sometime to make sure there was nothing wrong with my car. The dilema is: Looks vs. Mileage......."Things that make you go hmmmmmmmmmmmm"......
     
    #1 mike_m, Aug 12, 2006
  2. Ichabod

    Ichabod Artist In Residence

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    There are already so many quirky things about this car that I don't really mind the wheels making it look a little more like a toy. Personally, I would never dream of putting wheels like yours on my Prius because of the MPG hit. Maybe some day when I'm older I'll care less about the future and get myself a nice sports car, but for now I don't care if people think my car looks stupid, because I think they look stupid driving past me at 85+ mph.
     
    #2 Ichabod, Aug 12, 2006
  3. Scott_R

    Scott_R Member

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    At first glance, the subject heading of the thread looked like some of the spam I get.
    B)
     
    #3 Scott_R, Aug 12, 2006
  4. jimmyrose

    jimmyrose Member

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    Wow, that's a larger difference than I would have expected based on everything I've read here...
     
    #4 jimmyrose, Aug 12, 2006
  5. mike_m

    mike_m New Member

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    <div class='quotetop'>QUOTE(ScottR @ Aug 12 2006, 03:22 PM) [snapback]302203[/snapback]</div>

    Scott, look at the topic now! :lol:
     
    #5 mike_m, Aug 12, 2006
  6. Ichabod

    Ichabod Artist In Residence

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    Ha ha, that doesn't help much... how about this for a new topic name:

    Enhance your mileage! 7 MPG 0VERN1GHT!!! Your wife and the EPA will thank you!
     
    #6 Ichabod, Aug 12, 2006
  7. mike_m

    mike_m New Member

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    <div class='quotetop'>QUOTE(Ichabod @ Aug 12 2006, 04:25 PM) [snapback]302237[/snapback]</div>

    How bout this: NO :lol: :lol:
     
    #7 mike_m, Aug 12, 2006
  8. JasonQG

    JasonQG New Member

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    How does the Prius calculate how far it's travelled? I would assume that would be by the number of wheel revolutions. If that's the case, wouldn't the MPG calculations be off with bigger wheels, or is there an adjustment for that?
     
    #8 JasonQG, Aug 12, 2006
  9. Godiva

    Godiva AmeriKan Citizen

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    Sometimes there's a reason stock is stock.
     
    #9 Godiva, Aug 12, 2006
  10. ekpolk

    ekpolk What could possibly...

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    Mike:

    Try plugging your numbers into this tire size comparison calculator. The link is sometimes a tad slow (but it's stayed good for years), and when you get there, it's a bit cryptic. The idea is to fill in your tire data in the first boxes, hit calculate (actually called "compare tires"), and the comparative figures will appear in the display below. It's pretty surprising how far off you can get if you don't pick your upgrade sizes carefully. [​IMG]
     
    #10 ekpolk, Aug 13, 2006
  11. mike_m

    mike_m New Member

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    Well I did it again, just to see. I figured almost 3 weeks of the stock set up, I would put my bling wheels back on. I'll be darned if it didn't go right back to 39MPG. I changed back at fill up and reset the MFD. I was averaging about 45-46. It is so hard to believe that that really makes a difference. I figure it would have some "flywheel effect", and that whole "object in motion tends to remain in motion" thing. It doesn't. The engine does not labor with my 17's on either. It's knid of like the car knows they are there, and is protesting for no reason what so ever. I know all about the unsprung weight. It just amazing that something so small can do that. It's not like I hae a parachute hanging off the back of the car. (Maybe that would be better) :) I went to tire rack to see if I can find out how much the Pirelli Nero Zero's 215/17's weigh and there is no info. Does anybody know how much they weigh?
     
    #11 mike_m, Aug 30, 2006
  12. priusenvy

    priusenvy Senior Member

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    Assuming that the inflated replacement tire is the same diameter as stock, my guess would be that the dominant effect is increased rolling resistance, followed by increased moment of inertia. Larger diameter wheels not only weigh more (usually), but also put most of that extra weight farther away from the center of the wheel, further increasing the moment of inertia beyond that of just the increased weight. The higher moment of inertia requires more energy overall to get up to speed, and since the regenerative braking doesn't recover all the energy of the spinning wheels, you don't get all of it back when you brake to a stop.

    Again, that is just a guess, I can't back it up with any data. But I would measure the tire diameter again just to make sure that you're not really going farther than the car thinks based on its idea of your tire diameter.
     
    #12 priusenvy, Aug 30, 2006
  13. molgrips

    molgrips Member

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    The car thinks you've travelled less distance than you actually have, if you have bigger wheels. Hence much lower mpg.... surely?
     
    #13 molgrips, Aug 30, 2006
  14. mike_m

    mike_m New Member

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    <div class='quotetop'>QUOTE(priusenvy @ Aug 30 2006, 03:44 PM) [snapback]311726[/snapback]</div>

    <div class='quotetop'>QUOTE(molgrips @ Aug 30 2006, 04:17 PM) [snapback]311743[/snapback]</div>

    Thanks for the replies: According to the calcs. the stock tire is 824.5 revs per mile and my tire is 819.24 revs per mile. I went with the Toyota SE option tire, (215/45/17) and wheel. I think it's as close as you can get. I know it won't get any better. It was nice to put the stocks on to see that my car was capable of the great fuel mileage, since it came with the heavy combo.
     
    #14 mike_m, Aug 30, 2006
  15. GreenSpeed

    GreenSpeed New Member

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    Hey Mike!
    Yeah I'm the reverse actually, I just bought a set of tC take-off's from eBay and put it on my Prius. Definetly felt heavier carrying them, but also notice the mpg hit (engine comes on faster, slightly slower acceleration) but i think the looks totally make up for it. Cheers!
     
    #15 GreenSpeed, Aug 31, 2006
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