MPG and other changes with different tires.

Discussion in 'Gen 2 Prius Fuel Economy' started by '05 Prius, Nov 11, 2019.

  1. '05 Prius

    '05 Prius New Member

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    2005 Prius
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    '05 with 203k miles and a weak traction battery.
    Old tires-195/65-15. New tires-165/80-15. 0 miles, but over 5 years old.
    Test terrain- freeway with a slight grade, held 65-67MPH as well as possible.

    I ran both tires back & forth twice trying to get consistent numbers from the instant MPG display while fighting with traffic to maintain speed with as little change as possible.

    Old tires did about 32MPG one way and 42 the other. New tires did about 39 and 49. The numbers aren't perfect, I was being challenged by traffic. The new tires are a little taller than the old so the MPG is a hair higher than displayed. The new tires were at about 39PSI and the old about 36, I ran it straight from the tire shop to the 'test track', otherwise I would have adjusted the pressure before testing.

    The steering is noticeably lighter with the skinnies and it rides softer. Not sure on road noise, but it seems to be lower. Throwing it into a turn the new tires made more squealing noise more easily than the old, but I didn't push it hard enough to have a noticeable loss of traction. Accelerating from a stop into a right turn they seem to slip about as easily as the old but the traction control seems to do a better job with these, it didn't seem to loose power as harshly or as long. From the slip noise, feel, and traction control reaction these seem to start loosing grip a little earlier, but more progressively. The new tires are old so as they scrub off I think they'll get grippier, they only had about 10 miles when I traction tested them.

    It's my GF's daily driver, I'm super curious what she'll think of the change and curious to see what happens to the average MPG. I told her nothing about my thoughts on the tires and haven't asked her to report on them to not skew her impression. When she comes home tomorrow I'll ask if she noticed anything and I'll check the average MPG, I reset it after parking for the night. She usually gets around 38 driving it like a normal car. It seemed to rise a little after the recent oil change, but she also had some different than usual driving that she won't have tomorrow.
     
  2. dolj

    dolj Senior Member

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    Why did you go to 30 cm narrower tires?
     
  3. bisco

    bisco cookie crumbler

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    a new tire has a larger diameter, changing the readout calculation
     
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  4. Freefall Prius

    Freefall Prius New Member

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    Hello 05Prius, I have an 09 Prius with 145,000 miles. It's such a pleasant car to drive, even in it's squishy and lose state of suspension. Currently collecting a lot of suspension upgrades to add to the car. It will be fun to drive it then. When you say checking the "MPG Display," are you referring to the in-car on-screen readout? It's been my experience that my on-screen MPG average is not that reliable. So I'm strictly using the fill up at the station and divide miles driven by gallons gotten at the pump. Then of course, resetting the odometer each time I fill up. Currently getting 44 MPG. My onboard readout is usually off by 2 to 4 MPG on the high side. Just thought I'd mention that in case you want to try the gas pump method.
     
  5. TMR-JWAP

    TMR-JWAP Senior Member

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    The OEM stock tire size is 185/65/15 and is about 24.5 inches tall.

    The 195/65/15 is about 25 inches tall

    The 165/80/15 tire is about 25.5 inches tall.

    Each time the tire is taller, it travels further for the same rotation, so your actual mph and miles travelled is greater than indicated. Remember, your mph works off measured wheel rpm, so your true speed may now be 2-3 mph faster than the oem. It would just be a matter of crunching numbers for the tire circumference.

    The OEM is 76.97" circumference (823.17 rotations per mile)
    The installed tires are 80.1" circumference (791 rotations per mile)

    791 / 823.17 = 96.1%

    60mph indicated on OEM tires = 57.6 indicated on the 165/80/15 tires. OEM tires usually provide an indicated mph slightly higher than actual. You're going to be opposite that, which could lead to unexpected tickets, especially if you follow the mantra of "9 and you're fine, but 10 you're mine" when exceeding speed limits. If you show 70mph, you'll actually be doing about 73. You may want to let the GF know about that. She'll probably be pissed if she thinks she's rolling ok and then gets stopped by the po-po.

    Are you having any interference issues with the tire edges rubbing on anything at full lock?
     
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