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Tire pressure from dealer - 28

Discussion in 'Gen 3 Prius Main Forum' started by dragonrand, Oct 19, 2016.

  1. dragonrand

    dragonrand Junior Member

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    I just bought a 2012 Prius Four as certified used from the local dealer. One of my first stops was to check tire pressure and increase if needed. The tires are Yokahamas, and I was surprised to find them all at about 28-29. I was used to running the Firestone Ecopias on my Gen II at aboug 43/41, but I thought maybe the Yokas called for lower pressure. I examined the sidewall and found 40PSI there, so I aired them up to 39.

    I saw in another thread that the 40PSI spec is for something other than inflation, so now I'm thinking I can safely go back to maybe 43/41. Looking for advice on that, but my main questions have to do with the pressure monitoring system.

    • Does that require special valve caps? My Gen II came with those, but I replaced them with standard caps for reasons I can't recall now.
    • How low would my tires have had to be to trip the pressure alert? I'll never intentionally run them in the 20s where they were, and I'll probably rarely want to run them lower than the high 30s? How does the pressure monitoring system, work, and what are its thresholds?
    • I understand I can set the pressure where I want them and then reset the pressure monitoring system to that new "norm" If I want to be alerted if they drop below 40, would I have to deliberately overinflate the tires past the point where I intend to run them, then drop them down to compensate for the margin built into their thresholds?
    Thanks, just trying to get a handle on how it works.
     
  2. bisco

    bisco cookie crumbler

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    1) no.
    2) i think it's random.
    3) i think so, but not positive.

    i run mine at 42/40, but the tpms light doesn't come on unless they drop into the 20's. it's a good warning for a potentially flat tyre from a slow leak.
     
  3. wjtracy

    wjtracy Senior Member

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    Some folks say you can reset the TPMS pressure alarm point...I am going to fiddle with it next time I get Techstream running, as some also say you see the alarm set points.

    Tell you what, somebody needs to demonstrate this by posting the Techstream screen-grabs to prove it. I will do so but no promises on when I get around to it, as my Techstream compo is a desktop
     
  4. fuzzy1

    fuzzy1 Senior Member

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    Please go re-read that sidewall. The 40 psi limit is normally just for seating the bead during mounting. I don't recall any common passenger tires having 40 as the operating max pressure, normally it is 44 or 51.
     
  5. Mendel Leisk

    Mendel Leisk Senior Member

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    Considering their track record, better check the pressure on the spare too.
     
  6. lenjack

    lenjack Active Member

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    Spec is 35/33, front/back. Better mileage and handling at 4 to 5 over. Spare usually neglected, and should be at 60.
     
  7. Redpoint5

    Redpoint5 Senior Member

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    The tire doesn't determine inflation pressure, instead the placard in the driver's side doorjam of every vehicle specifies the inflation pressure.

    I run about 40 PSI for better MPG and steering agility.

    I wouldn't bother with TPMS. You'll know when your tires are dangerously low when the car begins to handle poorly. I'll check my pressures twice a year at winter and summer to adjust for changing temperatures. My tires might loose 2 PSI every 6 months.
     
  8. dragonrand

    dragonrand Junior Member

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    Yes, my old tires' max was 44. Looking more closely at my Yokahamas, I saw that their max is 51.

    All my tires seem to be slowly losing pressure at the same rate. 1 week ago I aired them up to 39/38. When I checked them yesterday evening, they were at 37/36. I aired them back up to 43/41, and checked them again this morning. They were already down to 41.5/40.

    The valve stems seem to be something more than the simple straightforward valve stems that I've seen all my life. They are heavier, metal, with a flare toward the base. These aren't valve stems that bleed pressure above a certain level, are they??

    If all my tires steadily lose pressure, it's no surprise that they were at 28 on delivery (except for the surprise that the dealer didn't recheck/verify the pressure).
     
  9. Mendel Leisk

    Mendel Leisk Senior Member

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    If you're adding air at a gas station, pressures are gonna be a bit higher from the outset. Then checking them cold, the next morning, it's natural for them to drop a bit.
     
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  10. exstudent

    exstudent Senior Member

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    NEVER EXCEED the cold tire max PSI that is imprinted on the tire sidewall. If your tires says 40 PSI MAX, that is the maximum cold PSI you can fill the tire. Remember, tires come in different speed ratings; faster rated tires willl most likely have a higher MAX cold PSI rating.

    1) Any schrader valve stem cap will do; bicycle, another car, decorative chrome cap, etc. The caps only purpose is to keep debris/water out.
    2) 25% difference from the last TPMS reset for Gen2. Factory door setting of 35/33, reset TPMS, warning will trigger at less than or equal to 26.25/24.75 PSI. Fill tires to what you want, reset TPMS, so the system learns the new threshold and adjusts the new warning trigger PSI. There is a high warning threshold (70PSI range IIRC), which is unlikely.
    3) Why make life more complicated? Just fill to what you want, reset TPMS, done. Sill check your tires regularly when COLD.
     
  11. fuzzy1

    fuzzy1 Senior Member

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    A cooling of 10-15 F would cause the same pressure shift.

    The pressure that matters is in the morning coolness, when the tires have not been run for several hours. The tires will warm both as the ambient temperature rises during the day, and as the tires run and flex on the road.
     
  12. exstudent

    exstudent Senior Member

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    Ideal Gas Law: PV=nRT

    Pressure x Volume = (number of moles of gas in question) x (R=Gas Constant of the gas in question) x (Temperature in Kelvin)

    Essentially pressure is proportional to temperature (driven or ambient). This also assumes you are not affecting Volume by changing the tire to a larger or smalle size. If is cold outside (say 50F), pressure wIll be lower. If its HOT outside (90F), pressure wil be higher. This assumes you have not driven, as the tires will be heated and have a higher PSI reading, than if they just sat.

    This is why it is important to check and adjust your tires based on the seasons: ambient air temps impact tire air pressure.
     
  13. fuzzy1

    fuzzy1 Senior Member

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    Note also that for this gas law, the pressure in the equation is 'absolute' pressure, which is roughly 15 psi higher than the 'gauge' pressure shown on the tire pressure gauge. The gauge subtracts out the local atmospheric pressure.

    This distinction is important only if you are trying to use the proportionality to compute how much pressure change is expected. Absolute temperature (K or R) is also needed, not the common civil scales (C or F).