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185/65/15 vs 195/60/15 tires - Differences in rotaional mass/intertia, rolling resistance, economy

Discussion in 'Gen II Prius Main Forum' started by Tekdeus, Jan 12, 2011.

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  1. Tekdeus

    Tekdeus Shifted to Green

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    I'm about to buy a set of low rolling resistance tires (Michelin energy savers) and I'm torn between the two size choices, 185/65/15 vs 195/60/15, and I'm trying to figure out the differences in rotaional mass/intertia, rolling resistance, and fuel economy.

    The 195/60/15 tires have a 117mm sidewall, while the taller skinner 185/65/15 tires have a 120.25mm sidewall (calculating 65% of 185 width). If I am calcuating this right, this means the the 195 tire has a 6.5mm smaller overall diameter than the 185 tire.

    Is there any way to calculate (approximately) which of these is likely to be more fuel efficient? Although the 195 tire has 10mm more rubber width on the road, its diameter is smaller, giving it less rotational inertia to get moving.
  2. uart

    uart Senior Member

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    Re: 185/65/15 vs 195/60/15 tires - Differences in rotaional mass/intertia, rolling resistance, econo

    Yeah the rolling circumference of the 195/60 tires is very slightly lower (around half a percent less) than that of the 185/65.

    In Australia 195/60 is actually the manufacturer recommended tire size. I cant compare them as I've only used the 195/60 size, but I don't think it has too bigger effect on fuel efficiency as I still get 4.2 L/100k (56 MPG US) with this size tire.
  3. donee

    donee New Member

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    Re: 185/65/15 vs 195/60/15 tires - Differences in rotaional mass/intertia, rolling resistance, econo

    Hi Tekdeus,

    I would just go with the lighter tire. In the specs page for your tires, on the Michelin website, they will give the weight of each tire.
  4. Hal W

    Hal W New Member

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    Re: 185/65/15 vs 195/60/15 tires - Differences in rotaional mass/intertia, rolling resistance, econo

    The 60 series is slightly wider ,is it not. If that is the case it would effect FE a little. How about 195x65 R15? Same width as OME but slightly taller.
  5. ct89

    ct89 New Member

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    Re: 185/65/15 vs 195/60/15 tires - Differences in rotaional mass/intertia, rolling resistance, econo

    There are many posts in this forum discussing tire sizes. Plenty of opinions to be found. In my experience, if you want your speedometer to be accurate, 195/65 R15 is very close to the best size.

    When running with 185/65 R15 tires on my Gen II, my speedometer consistently read about 3% too high when compared to my GPS. After I switched to 195/65 R15, the speedometer is within a fraction of a MPH of the GPS.

    Yes, with the slightly larger tire, the Prius will tell you that you are getting 2% lower mileage but in reality, it is the readings with the 185/65 tires that are inaccurate because with the smaller tire the car thinks it went farther than it really did.
  6. Tekdeus

    Tekdeus Shifted to Green

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    Re: 185/65/15 vs 195/60/15 tires - Differences in rotaional mass/intertia, rolling resistance, econo



    Hmm, the wieght is not listed: Energy Saver A/S | Michelin Tires It's not easy figuring out which would be more efficient.
  7. tortus

    tortus New Member

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    Re: 185/65/15 vs 195/60/15 tires - Differences in rotaional mass/intertia, rolling resistance, econo

    Everything else being equal, it seems to me that the smaller the foot print the smaller (less) rolling resistance.
  8. donee

    donee New Member

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    Re: 185/65/15 vs 195/60/15 tires - Differences in rotaional mass/intertia, rolling resistance, econo



    Hi Tekdeus,

    Tire Rack says the 185 is 1 pound lighter:

    Michelin Energy Saver A/S
  9. 2maples49

    2maples49 Junior Member

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    Re: 185/65/15 vs 195/60/15 tires - Differences in rotaional mass/intertia, rolling resistance, econo

    Tire Rack - Your performance experts for tires and wheels has these specs:

    Michelin Energy Saver A/S

    I suspect that if you want the lowest rolling resistance, get the stock size. I you want to improve the handling a little with a very small change ( I don't know how much ), get the 195/60 15's. About a 6" tread contact patch could be the sweet spot for handling and rolling resistance. Tires and construction are so complex it's really hard to tell. Low rolling resistance tires tend to be weak in the handling department, so I'd go for the 60 series.

    Enjoy your new tires, Dan
  10. Tekdeus

    Tekdeus Shifted to Green

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    Re: 185/65/15 vs 195/60/15 tires - Differences in rotaional mass/intertia, rolling resistance, econo



    Thanks. I went with the 185's, and will live with the skinner looking tire :)
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