PSI for tires rated with higher PSI.

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by SWprius08, May 9, 2011.

  1. SWprius08

    SWprius08 SoCalprius

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    So, I had upgraded my stock OEM integrities (185/65/15) which was rated at 44 PSI with the fantastic Goodyear Fuelmax (195/65/15) which are rated at 51 PSI max and am running these at 42 40 PSI. I took my car to the dealer for the 45K service (first time with these new tires), they changed tire pressure back to factory recommended 35 & 32. The car was dragging and the engine was on almost all the time I drove, MPG drastically reduced. Should'nt it be logically and engineerically correct to pump higher proportionate PSI on tires which are rated higher than OEM?

    Here is simple math (correct me if I am wrong)

    OEM Tire, Max PSI = 44. Factory recommended 35 (front) 33 (rear)

    Other Tire, Max PSI = 51.
    Front PSI, 35 x (51/44) = 40.57 PSI
    Rear PSI, 33 x (51/44) = 38.25 PSI

    Any thoughts?

    Disclaimer: Engineerically is not a dictionary word!!
     
  2. jdenenberg

    jdenenberg EE Professor

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    I run the Nokian WRg2's (also a great LRR tire) which are rated at 51 psi MAX. I still run at 42/40 psi as higher psi would result in a harsh ride.

    JeffD
     
  3. jdcollins5

    jdcollins5 Senior Member

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    Evidently most dealers are required to return the tire pressure back to the Toyota recommended pressure in the driver's door. I know I have to reset my tire pressure each time I take it to the dealer.
     
  4. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    While my Goodyear Integretys did well with 42/40, I can see that my Goodyear Viva Authority Fuel Maxs are wearing down the center of the tire, so I need to back off a bit. I suspect 38/36 for a while.

    [​IMG]
     
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  5. cyclopathic

    cyclopathic Senior Member

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    Go by feel more then max PSI. Higher PSI will give better MPG but if there is no grip and ride is harsh, why?
     
  6. SWprius08

    SWprius08 SoCalprius

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    jdenenberg, what size of tires do you have? I felt harsh ride as well with higher PSI, tried 46 44 and 48 46. I wonder if this is because of bigger tires like mine are 195/65/15?
     
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  7. Corwyn

    Corwyn Energy Curmudgeon

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    I don't see why. The maximum rating is based on the construction of the tire, and what the manufacturer is willing to warranty. The rating on the door panel is Toyota's recommendation based on whatever they base it on (probably biased towards ride rather than mileage; unless the run EPA tests at door panel rating). Although it probably considers different tires, it does so only as a compromise. I don't see any proportions at work here.

    So, what pressure should you put your tires at? Whatever works best for you. People on this forum have found that increasing pressure (up to a point) will give better mileage without causing uneven wear, though it will make the ride a bit stiffer. Why not start at door panel pressure and increase it every few days until you notice an unacceptable change in handling, or reach your comfort limit with respect to max rating on the sidewall?
     
  8. jdenenberg

    jdenenberg EE Professor

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    My WRg2's are the standard Prius gen2 size 185/65HR15. They are XL rated tires.

    JeffD
     
  9. SWprius08

    SWprius08 SoCalprius

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    I do not know if any one has tried this, take 2 tires one rated at 51 and another at 44. Pump both with 35 PSI. Which one will be softer and which one stiffer?

    Theoritically, I would think that the one rated at 51 would be softer. Again I could be drastically wrong. Please correct me if so.
     
  10. Mendel Leisk

    Mendel Leisk 'Orrible Oracle

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    It depends a lot on the tire. We had extra load Nokian WRG2's for winter on our last vehicle. At the same pressures as our OEM's they were a bit harsher.

    Bottom line: the dealership service department, in the absence of instruction, is going to set tire pressure to the placard in the doorwell, no matter what the tire. Depends on the dealership, but they'll likely leave them alone if you tell them to. They don't want to alienate customers.
     
  11. tonyrenier

    tonyrenier I grew up, but it's still red!

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    I tell them I run 42/40 and check them before I leave. I think it urks them but my mpg is great, tires are still fine at 24K. I'm still the customer, but they need reminding.
     
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  12. cyclopathic

    cyclopathic Senior Member

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    From playing with pressure on Gen III front should be no more then 40-42 PSI and rear 36-38. Above that grip will deteriorate. Granted exact pressure depends on tire wear, brand, load, gauge calibration, etc but what is the point to trade safety for MPG?

    On Michelin Energy Saver A/S with soft sidewalls 42 front/38 rear on empty works the best, on OEM Avids 40/36, YMMV
     
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  13. jadatis

    jadatis Junior Member

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    I once got hold of the calculations the European tire-makers use to determine the advice pressures, and worked it out.
    Learned myself Excell to make spreadsheets for it.
    Translated a few from Dutch to English to go worldwide with it.
    Recalculating tyre-pressure - Windows Live
    In this map spreadsheets and excamples to re calculate advice pressure for non OEM tires.
    Download and open the spreadsheets in Excell or compatible programm to use it and not directly in the browser , because that cant handle protection and data-validation I used in it.

    But in my search I first also made the mistake to use the maximum pressure of normal and extra-load car tires. This is not the pressure they calculate with. In Europe almost always the reference-pressure, as they call it is for normal car tires is 2,5 bar/36psi, and for extra load/XL/reinforced is 2,9 bar /42psi?. In American system 1 psi lower. So for your new tires you still have to use the 36 psi instead of the 51 psi, probably it arent XL tires.
    The extra is used for higher speed then 99 miles/h and camber-angle above 2 (alignment).

    What cyclopathic writes with the pictures only goes for diagonal tires wich arent used anymore, a radial tire keeps the wide of the tire over a large range on the ground, only the length gets less or more.

    If you are interested I can write you more , and from the link I gave you can navigate my complete map of hotmail for more , yust like in a forum.
     
  14. SWprius08

    SWprius08 SoCalprius

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    Jadatis, I wish I can spend more time understanding the formulas in the excel file. If you can write more I am sure it would be helpful.
     
  15. dhanson865

    dhanson865 Expert and Devil's advocate

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    FWIW I've put 6 or 7 thousand miles on my Yokohama AVID ENVigor (H) 185/65/15 tires in just over a years time. I've had them at or above 50 PSI once or twice but am keeping them in the 40s now.

    My logic for keeping pressure is

    If I know I'm doing short commutes on a regular basis (like 2 miles each way to work) with no time for the tires to heat up I do 50/47.

    If I don't know my average trip or know I'll be taking a longer trip I'll set it up for 48/45.

    If I think it might snow I'll lower it to 45/42 (at 50 to 60 degrees in my garage) and let it shrink how ever much it will shrink when it gets below freezing but that's a rare occurrence where I live.

    I haven't checked the tire pressure lately but will again in a week or two when the temps drop. I checked the tire pressure often in the first six months and found the pressure hadn't changed even 1 psi for the first few months. Eventually I put more air in a couple of times over the last year but it's never been much lower than my target so even at the lowest its staying in the 40s.

    So far I haven't had any complaints about traction/handling and I haven't noticed any uneven wear. I haven't even rotated the tires yet (I might rotate them soon but I'll definitely do it before the 10K mark). I'm sure the fronts have worn more than the rears but without taking a tread gauge to them I wouldn't know (the dealer takes a tread gauge to them when I get the oil changed and that is how I know the front has more wear).

    Supposedly at 80,000 the tread depth was 10 all the way around, and at 82500 it's 8 on the fronts and 9 on the rears. The short interval wasn't an oil change for both, one was a warranty water pump replacement for free. I figured it was a good week for me to do it so 82K+ was when it happened.

    So if my tires started at a tread depth of 10 or 11 and they are down to 8 would you expect the uneven center wear to show by now? If not how much longer would it take to show up?
     
  16. SageBrush

    SageBrush Senior Member

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    I figure any pressure between car manufacturer recommended and maximum allowed on the sidewall is fair game. I run the front at max sidewall in the winter, and 2 psi lower in the summer. Rear are 2 psi lower than whatever the front tyre pressures are set.

    My current tyres are Michelin hydroedges, and at ~ 50k miles are wearing down even slower than the 90k mile warranty allows. I do rotate about twice a year, around 7.5 - 10k miles.
     
  17. xs650

    xs650 Senior Member

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    Reported tread wear numbers from shops are notoriously inaccurate. If your front tires actually wore 2/32 in 2500 miles they would be worn out at 10,000 miles.

    I think the average shop guy guesses at tread depth unless they are nearly worn out and he smells a potential commission from selling you some tires.

    The only way to meaningfully track tire wear is to do it yourself.
     
  18. dhanson865

    dhanson865 Expert and Devil's advocate

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    Agreed and I intend to monitor the tires myself. My question was what minimum amount of mileage would I have to put on the tires before I should be able to easily see a difference in center wear vs edge wear due to over inflation?

    My contention is that throughout the life of these tires I'll keep them in the mid to upper 40s and they won't show any uneven wear. If I'm wrong I'll back down the pressure a little but if I'm right I'll keep it high.
     
  19. oldasdust

    oldasdust Member

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    An old timer who everyone called a tire guy once said with modern tires run at least 80 % of max pressure printed on the sidewall without a loaded vehicle. Increase with a load.That is a starting point, but it all depends on personal preference ride, mpg.
     
  20. xs650

    xs650 Senior Member

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    Most people running the the 40 something psi range don't experience uneven wear, a few do. When you experience it will depend on how bad it is, if it happens at all.

    Radial ties usually handle extra pressure very well. That's because the circumferential belts tend to keep the tread flat across the tire over a wide range of pressures.
     
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