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Smaller sidewall tire size

Discussion in 'Gen II Prius Accessories & Modifications' started by Daves09prius, Sep 8, 2011.

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  1. Daves09prius

    Daves09prius Active Member

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    185/65R15 vs. 185/55R15

    The OEM tire size for my 2009 Prius Base is 185/65/R15
    I don't have the money to buy larger rims and tires like I really want to, however I need new tires. I can go a bit longer on them, but they will need to be replaced soon. And since I have to get them anyway, why not upgrade to something better!

    Has anyone tried a tire with a smaller sidewall to improve handling and turn-in? I was looking at 185/55 R15's on tirerack.com and they are about the same price as the factory OEM size replacements.
    Has anyone tried using a tire with a smaller sidewall?

    I'm rolling over on the sidewall while taking sharp curves, and accelerating on the on ramps as I drive. I know that the smaller size will change my speedometer reading, but I don't really care about that. I just wonder if someone has tried it, and did it work?
  2. F8L

    F8L Protecting Habitat & AG Lands

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    It will work just fine except your mpg reading will get screwed up as well as the odometer. Anything over a 3% change in size is not recommend. If you do change the size then monitor your speed via gps so know how many mph numbers to subtract from your speedometer.
  3. fuzzy1

    fuzzy1 Senior Member

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    Have you already increased the tire pressure up from the door placard label, up to your comfort limit?

    If not, more pressure will help. But I can't say that it will help enough at the maximum pressure you dare use, for either ride harshness or safety.
  4. Daves09prius

    Daves09prius Active Member

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    I've boosted the tire pressure up to 44 Front / 42 Rear PSI and I've seen the wear pattern change, it's not wearing out the outer edges of the tires anymore. Now it's more even. The recommended pressure has the wear pattern of an under inflated tire. Hopefully increasing the tire pressure will help me get another few months out of them before I absolutely have to buy new ones.
  5. Pinto Girl

    Pinto Girl New Member

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    In addition to the other comments, it'll also make the car look a little funny since the tires won't fill the wheel wells as they do now.

    Have you looked for rims from a wrecked Touring? That's what I did, and they weren't super expensive. F8L is right though, this mod made the speedo read slow. But most cars tend to read 2-4 mph fast anyway; I discovered that it was more accurate than before since the indicated speed wasn't always a bit high.
    Daves09prius likes this.
  6. dhanson865

    dhanson865 Expert and Devil's advocate

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    Instead of getting a different size tire consider getting better tires. I put Yokohama AVID Envigor on my 2005 and pumped the tire pressure up higher knowing it has a 51 PSI sidewall rating.

    If you don't care about the difference in MPG/speed/odometer displays you could also consider the General Altimax HP. I put them on my Saturn but they have a different rated revs/mile.

    According to Tire racks data the revs/mile are

    Goodyear Assurance Fuel Max 855
    Goodyear Integrity 855
    Yokohama AVID ENVigor 850
    General Altimax HP 843


    So I took the AVID over the Altimax just to make the displays more accurate at stock tire size. But either has way better performance characteristics vs the OEM Integrity tires.

    And if you go for 45 front 42 rear on the PSI the rolling resistance difference will be minimal vs the Integrity tires.

    I'd shop on price between the three and then assuming you don't balk at the difference in price go for

    Altimax HP for better snow handling if you don't swap tires for winter/summer.

    Yokohama Avid Envigor for better cornering, dry traction, and longer treadwear.

    Goodyear Fuelmax for a stated Low Rolling Resistance.

    I can't think of a reason to buy the Integrity.

    As to what the difference in accuracy is: If the computer uses 850 then the difference is less than .5% for either direction. All of this is assuming your Prius has 15" rims. There may be other choices if you go 16" rims.

    edit: F8L mentions in the post below this that (I'll round to the nearest 5) 845 might be the correct target for revs per mile. With that we get

    Stock 855 1% too fast on the display
    Avid Envigor 850 .5% too fast (practically dead on)
    Altimax HP 843 perfectly accurate.

    I don't know if 845 or 850 is the correct target. It's possible F8L just doesn't notice a .5% difference?
  7. F8L

    F8L Protecting Habitat & AG Lands

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    As far as accuracy of the speedo is concerned, Pintogirl is right, the speedo is off by 1-2 mph with the factory Integritys. A taller tire actually corrects the speedo error.

    My Michelin MXM4 215/45/17 tires are taller than my factory 15s with revs per mile of 844. My speedo used too read 2 mph too high and now it is almost perfectly accurate.
  8. dhanson865

    dhanson865 Expert and Devil's advocate

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    It'd be nice to know the official spec. I wonder if one of the priuschat engineer types knows by way of the service DVD what the correct revs per mile target is?
  9. F8L

    F8L Protecting Habitat & AG Lands

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    I wonder if the service manual even goes into that much detail. My girlfriend's Rav4 has slightly taller tires on it and when I questioned her about it she said that is the tire the tire dealer or Toyota dealer recommended even though that is not what her car comes equipped with from the factory. So I decided to check the speedo via GPS and it was DEAD on accurate. Since most cars display MPH 1-2 higher than actually I guess that using a taller tire than factory and seeing the speedo become accurate is not so unusual.
  10. comtech5

    comtech5 New Member

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    So if I understand this correctly, if I purchase new 17" rims, I can go with Altimax HP 215/45/17 and the speedo and mpg will actually become more accurate? As well the handling and safety will improve?
  11. F8L

    F8L Protecting Habitat & AG Lands

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    It is likely yes. The tire you listed has a revs per mile of 839. My tires have a revs per mile of 844 and I am almost dead on accurate when compared to a GPS.
  12. dhanson865

    dhanson865 Expert and Devil's advocate

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    There is debate on what is the correct RPM (revs per mile) / TPM (turns per mile) for the gen II prius. The OEM 15" Integrity in the US is 855 RPM.

    Some members say by way of GPS that the speed is closer to accurate at 850, 845, or even less. But I've seen no mention of testing in a way to rule out innacuracy of the GPS. No mention of mile marker or fifth-wheel testing. But still the GPS is likely a valid clue though I wouldn't just synch to it as accurate.

    Worse factors other than the brand/model of tire affect RPM. From full tread depth to baldness, speeds can vary by up to about two percent, or 1.4 mph at 70 mph. Lowering tire pressure 5 psi, can result in about half that difference. Overinflation or oversize tires slow down the speedometer.

    So is 855 the correct tire spec only if you run 35/33 PSI as toyota recommends? If 40/38 is good for 1% difference and you plan to run higher PSI then that alone makes your target 846 RPM.

    In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226, which is pretty lax. To begin with, manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph. And those percentages are not of actual speed but rather a percentage of the total speed range indicated on the dial. So the four-percent allowable range on an 85-mph speedometer is 3.4 mph, and the acceptable range on a 150-mph speedometer is 6.0 mph. You're allowed another plus-or-minus two percent near the extremes of 20-to-130-degrees Fahrenheit, and yet another plus-or-minus one percent if the gauge was ever exposed to minus-40 to plus-185 F. Tire error is excluded from the above, and odometer accuracy is more tightly controlled to plus-or-minus four percent of actual mileage.

    The European regulation, ECE-R 39, is more concise, stating essentially that the speed indicated must never be lower than the true speed or higher by more than one-tenth of true speed plus four kilometers per hour (79.5 mph at a true 70). Never low. Not even if somebody swaps a big set of 285/35R-18s for stock 255/45R-16s.

    with credit portions of this info taken from a 2002 article from Speedometer Scandal! - Feature - Auto Reviews - Car and Driver

    I was surprised to see that they can modify the output of the speedometer so that it isn't a linear scale but I had suspected as much from prior experiences.

    So if you wanted Alimax HP tires on a Gen II Prius I'd consider

    185/65/15 if you will run the PSI up in the 45-50 PSI range
    195/60/15 if you will run the PSI in the 35-40 PSI range
    205/60/15 if you will run the PSI at or above 50 PSI

    I personally wouldn't go for the 17" rims but

    215/45/17 has similar revs per mile to the 185/65/15 in this tire so it'd be good in the 45-50 PSI range by the assumption that 855 was the OEM target and PSI matters to revs per mile. I guess the question to ask is what PSI are the 215/45/17 people saying makes the tire accurate at what speed?
  13. F8L

    F8L Protecting Habitat & AG Lands

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    In my particular case, I observe a 2mph over speed with the 185/65/15s. This was the case with my Yokohama dB Super E-spec tires and the Nokian i3s. Both tires were set between 44psi and 50psi.

    With the Michelin Primacy MXM4s with tire pressure at 44/42psi I observed approx a .5mph - 0mph over speed.

    We could try to get very scientific about this but I feel that it is easier to just choose the best tire for your circumstances and worry about the speedo later. It is easy enough to purchase your desired tire then test your speedo against a quality GPS unit in an area with excellent satellite coverage. If the speedo is off it is then easy to just add or subtract 1-2mph from your speedo and drive accordingly. :)
  14. comtech5

    comtech5 New Member

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    I just read several posts that the 215/45/17 is a great choice but the increased rolling resistance will cost me an average of 6mpg! That's way to much to lose just for appearance and handling. My original goal was to improve the safety on wet roads. So I am now considering just increasing the width a tad to 195/65/15 (831revs).
  15. F8L

    F8L Protecting Habitat & AG Lands

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    I think that is a good choice but keep in mind that the loss with the 17's really only cost you $120-$180 in fuel per year depending on your annual mileage. That is how I justify my loss. Although if you care more about fuel use for environmental, social, or political reasons then you would be using an extra 40ga each year based on 15,000 miles annually.
  16. Britprius

    Britprius Senior Member

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    In the UK I have fitted 205/55/16 tyres (OEM being 195 ) with great results. I can only imagine how poor 186/65/15 must feel, and would recommend to go to perhaps 205/60/15 even if you take a small penalty with MPG witch with careful choice of tyre I believe you can avoid. Perhaps Kumho KR22, should last a long while with UTQG of AA800 LRR tyre.
    Daves09prius likes this.
  17. dhanson865

    dhanson865 Expert and Devil's advocate

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    Just to be clear, when you say the dB super E spec were 2 mph over you mean the Prius would show 62 MPH when the GPS said 60 MPH?
  18. F8L

    F8L Protecting Habitat & AG Lands

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    That is correct. This also one of the reasons that some people observe a mpg.loss with new tires. They may be driving 2 mph faster than there were when driving on the older worn tires or smaller diameter tires.
  19. Andy FI

    Andy FI Newbie Gas Saver

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    Hey!

    In Europe we get 195/55/16 V as standard. Just swapped my original Bridgestones to Continental (Original BMW supplier) and love the ride. Much firmer and no significant impact on MPG noticeable, at least based on the first tank. Don't count too much on this as I'm still learning - a lot.

    Attached a pic.


    Cheers from a newbie :)

    Attached Files:

  20. Daves09prius

    Daves09prius Active Member

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    I've been reading that 205/60R15 is a good option to consider. I've found 205/60R15-91H General AltiMAX at Wal-mart for $77 each ($308 + Hazmat and Tax - NO CHARGE FOR INSTALL!!!) As much as I hate shopping at Wal-mart for tires, Tire rack had great reviews and ratings for this tire. So a tire upgrade for less than $350 Hell yeah I'm going to go there and buy the tires!

    Link to youtube videos:


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