Gas milage drop with new tires

Discussion in 'Gen 2 Prius Main Forum' started by stl1rab, Nov 14, 2007.

  1. stl1rab

    stl1rab New Member

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    I have a 2004 prius that came with Integritys. They wore out pretty quickly so I bought some 75000 mile Firestones. My milage dropped from 50/51 to 39/40 on a good day. I had the tires two months and read a post on here from someone that they got Goodyear Viva2 and their milage went from 50 to 60. So after only putting 7000 miles on my new tires, I bough a new set of Viva2. Now my milage is at about 43. Am I missing something? I'm thinking of getting a new set of Integritys if I can find them. This is becoming quite annoying with gas prices going up and I drive 125 miles a day to get to work and back. Any advce or help is appreciated. Thanks. Also if anybody needs a set of good Firestones, they are in my garage collecting dust.
     
  2. JimboK

    JimboK One owner, low mileage

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    Though the Integritys are not the best tire in the world for handling and tread wear (IMHO), as a low rolling resistance tire they're among the best at fuel economy. I seriously doubt whether any tire would add 20% to someone's fuel mileage after switching from Integritys, and most will cause a drop, as you've experienced. I estimated a 5-10% drop after I switched to Michelin Hydroedges.

    Another factor at work is that new tires will do worse than well worn tires of the same model. Give yours some break-in time and you should see an improvement. All things equal that is. Don't expect it to go up as temperatures go down, for example.

    Your decision of course, but I wouldn't buy yet another set hoping for another small increase. And I'm saying that as someone obsessed with fuel economy. Yes, gas prices are going up, but it doesn't make sense -- for economic and other reasons -- to buy a new set every 7000 miles or so.
     
  3. F8L

    F8L Protecting Habitat & AG Lands

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    That seems like much too large a drop.

    Even swapping from my 17 rims and MUCH wider/sticker tires I only saw about a 5mpg drop give or take. My rim/tire combo was 6lbs heavier as well.

    I would think that to correctly assess the situation you are going to need a larger sample rate. You are going to have to analyze it over a couple thousand miles. This will allow the tires to break in and factor out any errors due to climate change, driving habits, wind and road conditions etc.

    If you based this on one singe tank then wind speed/direction alone could have created the drop in milage. :)
     
  4. Marlin

    Marlin New Member

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    <div class='quotetop'>QUOTE(stl1rab @ Nov 14 2007, 01:49 PM) [snapback]539514[/snapback]</div>
    Do you monitor your tire pressure? I'd bet they were set at 30 psi by the tire shop. That's 15-20% low for the Prius.
     
  5. kingofgix

    kingofgix New Member

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    Ok, have some good info for you. I have tracked every tankful since I bought my '04 Prius and I currently have 56,000 miles. I calculate long term mileage using the "error free" method - total miles driven/total gallons purchased. The first 38,000 mi were with the orignal Integritys and I averaged 51.5 mpg, running at 42 psi front/40 psi rear. At 38,000 miles I purchased a set of Michelin Hydroedge. With the Michelins I have averaged 50 mpg running the same pressures. The Michelins handle much better and have better grip on on wet and snowy roads. I would recommend them, but there will be a slight mpg hit over the Integity tires.

    The type of wide swings in mileage you mentioned in your post cannot be real long term numbers IMO. To see those types of differences there have to be several factors coming into play, and you aren't comparing "apples to apples". I can see those sorts of differences from tank to tank, but tank to tank mileages cannot be used to compare tires.
     
  6. apostasy10

    apostasy10 Junior Member

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    <div class='quotetop'>QUOTE(stl1rab @ Nov 14 2007, 12:49 PM) [snapback]539514[/snapback]</div>
    I'm in a similar situation: I didn't think about what kind of tires they were going to replace my last set of tires with (and kindof assumed they would use the same _kind_ of tires) and suddenly I'm getting 48 mpg instead of 53. (I often can't even get 50 mpg going as slow as 55 on the highway.) It didn't occur to me until recently that it was the tires that was hurting my gas mileage. Has anyone ever tried to sell a set of gently-used tires? I did some quick math and it wouldn't make financial sense to buy a new set of tires if I got $100 for my used tires, but I like knowing I can get 50+ mpg when I am willing to driving efficiently.
     
  7. David Beale

    David Beale Senior Member

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    The best way to understand what happens with new tires is to watch a NASCAR race. New tires give a lot more grip for a while. Grip equals friction, which is drag. That lowers mileage. Once you wear off the "newness" the grip decreases, lowering drag, and the mileage returns. Unless you got tires that have a lot more grip and are not anywhere near LLR types. Larger tires will give more grip, so lower mileage. They may also push more air, lowering mileage.

    When I put on the Nokian WRs I didn't notice much of a drop in mileage two months ago. Difficult to tell now, as the temp has dropped, and with that so has the mileage. My Prius is turning into a "gas guzzler", getting around 5.2 l/100 km or so (45 MPG US). Well, it seems so when you're used to 4.2. ;)
     
  8. NoMoShocks

    NoMoShocks Electrical Engineer

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    I bought my Nokian WR tires on Tuesday this week, and I have about 120 miles on them. I LOVE my new tires. I know it is kind of early to tell, but comparing the same route from one day to the next, my standard commute, I have seen no drop in mileage at all, even though the WRs are not broken in, and the pressure is at the 35 psi from the tire store, rather than 42/40 psi I was running on the integrities.

    Nokian WRs are traction rated snow tires with a snowflake emblem on the side so you don't need tire chains unless you want them. But they also have summer capabilities in that they run quiet and have a 50,000 tread life rating.

    I upsized my to 195-60R-15 which weigh 18 lbs. and 860 rev per mile.
    If I has stuck with 185-65R-15, they would be 17.4 lbs and 851 rev per mile.
    Integritys are 855 rev per mile and 17 lbs.

    I am SO HAPPY I bought the Nokian WRs. They handle so much better and I have not felt any slippage yet. Last day with the Integrity tires, they were slipping all over the place and there was no way I would risk my Prius or my life any longer.

    One thing you may want to watch for is that their is a new tread desing Nokian WR-G2 being phased in right now that may be slightly better. I got the older design, becuast they were there.
     
  9. TomClement

    TomClement New Member

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    Another factor that you need to consider is the diameter of the tires. I bought some new Michelin Hydroedge tires recently and was really disappointed to see my mileage apparently drop by about 5 MPG. This after I made sure to inflate the new tires to 41 PSI.

    That was pretty worrisome, until I realize that these 90K tires have a much thicker tread and probably have a larger circumference than the ORM tires. If the circumference is larger, the spedometer won't know it and will report that you're going slower than you are, and the computer that calculates milage will make an equivalent error.

    I suppose they are also heavier around the edges creating a flywheel effect that probably also adds to the effect.

    I haven't measured the difference between my almost worn out OEM tire circumference and the new Michelin's. Does anyone out there know if this could account for a 10% reduction in apparent milage?

    Tom
     
  10. KTPhil

    KTPhil Active Member

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    For tires in the Prius size range, a change of 1/2 inch in tread depth would be a little less than 5% change in circumference. That is thicker than most treads, so it has to be more than just tread depth, though almost half the observed change in mpg would eb explained by this effect.

    I think the tires also wear-in based on position on the car. Rotating can give a smaller change in MPG for a tankful or two, also. Coupled with the stickier new compound, maybe this all adds up to the 10% drop.
     
  11. Qlara

    Qlara New Member

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    Check your alignment too?

    Suspension and bushings will degrade over mileages and time, which may make your brand new tires seems to drag more. So the best time to give your alignment a check out is when the new tires are on.
     
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