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One reason indicated mpg looks better as car breaks-in

Discussion in 'Gen 3 Prius Fuel Economy' started by Aegison, May 23, 2010.

  1. Aegison

    Aegison Member

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    A lot of other threads have noted that mpg seems to get better as the car gets broken-in. One reason why -- for the mpg indicated by the car's computers -- is tire wear.

    I while ago, I posted a thread which I'll copy in below. It had to do with the indicated mpg reduction which results from getting new tires.

    Well, it just hit me that the reverse must be true too -- namely, that as the tires wear, indicated mpg will tend to increase on the car's displays.

    To make sense of it, read that first post:

    ---------------------------------------

    Interesting analysis of MPG effect of replacing worn tires with new ones.

    Excerpt:

    Conclusion
    While many of these individual differences may seem insignificant, it is easy to understand that when they are added together, the new tires may appear to reduce vehicle fuel economy. It also means that a Toyota Prius appearing to get 50.0 mpg just before replacing its worn-out tires with new tires of the same brand, type and size, might be reduced to registering just 47.25 mpg afterwards, even if all of the driving conditions were identical.
    Remember, "your mileage may vary."

    Full article is at:

    Tire Tech Information - Tire Rolling Resistance Part 3: Changes to Expect When Switching from Worn-Out to New Tires
     
    #1 Aegison, May 23, 2010
    1 person likes this.
  2. donee

    donee New Member

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    Hi Aegison,

    Tire diameter change does not explain the improvement in mileage a few thousand miles after gettting new tires. I think the side-walls become more compliant, and generate less heat.
     
    #2 donee, May 23, 2010
  3. Tideland Prius

    Tideland Prius Moderator of the North
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    Overall, yes. (May also explain my 60mpg record tank but the calc'd figure was dead on as well... averaged over 3 tanks, it didn't deviate.. this is with the Gen 2 Prius).

    The larger difference in mpg btwn new and old tyres is also the new tread, in addition to the larger diameter because it's not worn now, all else being equal of course.


    This "effect" is also prevalent when someone test drives a new car. A new car's suspension is gonna feel different than a "worn" one even if it's the same car.
     
    #3 Tideland Prius, May 24, 2010
  4. Mike Dimmick

    Mike Dimmick Active Member

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    The tread depth on new tyres is about 7mm, which on the circumference of the 15" tyre is worth about 2.2%. (A bit more on the 17" whose overall diameter is smaller.) My understanding is that the tread depth on higher speed-rated tyres is shallower to stop it moving around so much, which causes heat problems at higher speeds, but obviously this causes the tyre to wear out quicker.

    However, I wouldn't think that you wear enough tread off the tyres just through 'breaking in' for this to be a significant effect.

    More likely, if there is any tyre effect, it's that new tyres still have remnants of the products used to remove the tyre from the mould, and this reduces the grip level somewhat. Once you've run them for a while it wears off and the tyre grips better.
     
    #4 Mike Dimmick, May 24, 2010
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