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So 40 psi == bad idea? (Dealer says so.)

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Troy Heagy, Jul 3, 2014.

  1. Troy Heagy

    Troy Heagy Member

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    Took my prius for an oil change, and told them not to touch the tires (or air filter... they try to sell me a new one even when it's still young). The tires were 40 but the mechanic reset them to the door label & gave me a mini-lecture about "Tires will pop if you take them out to the interstate, because the air will get hot & blow up. They are only rated 44 psi maximum."

    On the advice of Wayne Gerdes & other hypermilers, I've been setting my Honda Insight (1999 variant) to 40 for over a decade, and they never popped (official rating is 38/35). Of course the insight doesn't weigh as much as a Prius (1900 pounds). Perhaps I should listen to the dealer's mechanic?
     
  2. phoenixgreg

    phoenixgreg Senior member

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    I've been running my Primacys at 40 (both front and back) for a year with no trouble. Have crossed a scorching hot desert - no issues at all.
     
  3. Troy Heagy

    Troy Heagy Member

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  4. CLUBGUY

    CLUBGUY Member

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    Generally speaking the tire pressures posted on the vehicle door is a figure the mfg. has determined offers a good compromise of ride comfort, handling/road holding and fuel economy...
    Most highway tire pressure related failures result from UNDER inflation leading to extreme sidewall flex and tire failure....
    If the increase you mention does not cause a too harsh ride for your taste; there is no reason NOT to keep the pressure figures you mentioned...
     
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  5. Jon Hagen

    Jon Hagen Active Member

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    The mechanic is full of it. That max inflation pressure on the tire is a COLD max pressure setting, nothing at all wrong with running them at max pressure if your tread wear is even across the tire face.

    I have always(3 years) run my Goodyear Assurance Fuel Max tires at the cold 44 psi with no problems at all. The few PSI pressure rise as the tire heats up is calculated into the max cold pressure recommendation, no problem.
     
    #5 Jon Hagen, Jul 3, 2014
    Last edited: Jul 4, 2014
  6. KennyGS

    KennyGS Senior Member

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    My dealer sent me out the door with my tires aired to 40+ psi on a brand new Prius. I've since lowered them to the recommended pressures on my door jamb because I want a softer ride over higher mpg.

    I also run higher pressures (near 40 psi on the fronts) on my Corvette when running it on road courses.

    Your tires will probably not blow up.
     
  7. retired4999

    retired4999 Prius driver since 2005

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    Try a different dealer, they do not know what the H*** they are doing!
     
  8. 3PriusMike

    3PriusMike Prius owner since 2000, Tesla M3 2018

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    Not only is the max pressure based on a COLD reading, it is based on some (relatively) big pressure gauge error causing a lower reading than actual. (IIRC it is 3-4 psig at those pressures). And based on long continuous driving at high speeds in Death Valley temps, etc.

    Mike
     
  9. FroggyTaco

    FroggyTaco Member

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    Lol...the ignorance will never cease to amuse me.

    Probably a sincere & well meaning caution but not accurate.
     
  10. jonb505

    jonb505 Member

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    My Toyota dealership always sets my tire pressures as I request when i take it in for service. Currently running 40 front 38 rear(might try 38/36 next time), never even check them myself anymore. I'm quite certain they set them as i requested. I can easily tell the difference between 40 psi vs 32 psi when i hit a pothole. :)
    Then again my dealership seems to always up sell me on some fuel/oil additive or some other "recommended" service, so maybe they're just being agreeable to keep the cash flowing from my wallet. :cautious:
     
  11. orenji

    orenji Senior Member

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    You will have less rolling resistance with the tires PSI set high, this will also offset the extra stress your engine will have to work (decreased RPM) to maintain your speed at 80 MPH!
     
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  12. dorunron

    dorunron Senior Member

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    I too have to disagree with your dealer/mechanic. As stated the warning on the side of the tire for max pressure is for a COLD measurement. The manufacturer of the tire has researched and designed the tire to operate normally under the most extreme conditions when fully inflated. They even allow a margin of error due to variations in air pressure gages. As stated earlier, 99% of all tire failures is due to the following. Also as stated above the pressure's on the tire placard on the car is a "minimum" air pressure. It is calculated by the manufacturer (Toyota) to give you a cushy ride, fair tire wear and decent economy. The fact remains that increasing the tire pressure will cause the tire to have lower "rolling resistance" therefore the MPG should improve with the additional air pressure. Some have also found that running at the placard pressure will actually wear out the inner and outer tread before the center is worn out. This is typical tire wear of "under inflation".

    The following is the chief cause of failure of tires on the highway.
    1. Tire Under Inflation
    2. Abuse of said tires (brake jacking, peeling rubber, etc.)
    3. Use of tires when the tread or sidewall has been damaged. - It is suggested to check your tires physically for any visual damage and also to maintain said air pressures. You should check your tires at a minimum of once every 30 days. I myself monitor air pressure on a weekly basis, and sometimes even more than that. If I see a major fluctuation in MPG on the readout I immediately check all four tires. I maintain 41 front, 40 back at this time. The Michelin Primacy MXM4 I am running has a max of 51 on it. I have yet to fully inflate those tires. But I have no doubt I can do so and safely drive at high speeds on the hottest pavement out there.
    4. Not maintaining your tires. This includes checking the air pressure, along with rotation on a regular basis. Checking for even tread wear across the face of the tire. Checking for damage to the tire treads and the sidewalls. Keeping your vehicle properly aligned. And lastly making certain that there is no damage to the wheels that the tires are mounted on as well as the lug nuts being securely tightened and torqued to the specified foot pound rating.
    5. Debris on the highway. When driving at high speed if you strike something with enough force to cause the tire to rupture, you can have a blow out. When this happens it can be difficult to maintain control of the vehicle. Therefore drive sensibly and keep your eyes on the road at all times. At high speeds, you will approach things very quickly. You may not have time or space to adjust your path to avoid striking a hazard. Rubber is soft, and can be cut fairly easy by many different objects.

    It is not difficult for a individual to do all five things above. It only takes a few minutes to do this stuff. It could save your very life.

    Ron (dorunron)
     
  13. vvillovv

    vvillovv Senior Member

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    I thought I read 38 in one of mine yesterday, but today they all are 32.
    must be used ICE oil on my glasses.
     
    #13 vvillovv, Jul 4, 2014
    Last edited: Jul 4, 2014
  14. -1-

    -1- Don

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    :)I think it's fair to say that many techniques we learn here are proven and safe. I see no safety issue when I inflate (not over inflate) the tires on my Prius to 40/38 psi. Like a well conditioned athlete, I'll use every advantage to increase my MPG. With exception, I prefer 17" wheels, while not excessive, for a better look.
     
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  15. KennyGS

    KennyGS Senior Member

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  16. Easy Rider

    Easy Rider Active Member

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    That so called "mechanic" is a moron.

    The pressures listed are COLD pressures.
    If you drove it in for service, they probably were at least warm and SHOULD have been somewhat above the cold recommendation.

    By putting them back to the cold recommendation when they were hot, he has caused them to be UNDER-inflated when cold.

    I think going up the the max. listed on the sidewall is overboard but 40 is fine.
    It will make the ride a tad bit harsher, save a tiny bit of gas, wear the suspension a tiny bit faster and maybe cause a little uneven tire wear, depending on what tires you have.

    It's more a matter of personal preference than of getting any significant gains.
     
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  17. retired4999

    retired4999 Prius driver since 2005

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    I run mine 42 ft and 40 rear and for almost 10 years on Prius cars no! I repeat no! tire problems of any kind. Even across the face and no front end problems I know of. YMMV.
    32 psi compared to 42 psi.
    Mpg will Go up about 5 mpg or so. YMMV.
     
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  18. Mendel Leisk

    Mendel Leisk Senior Member

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    I keep our low profile OEM's (215/45R17) around 37 psi, and snows (195/65R15) around 40. I have them moderately high, but have a couple of concerns that keep me from going higher:

    1. Especially with the low profiles, I feel like a blinking bobble head going over parking lot speed bumps. Ride does suffer at higher pressures.

    2. I've got a suspicion higher pressures will shorten shock absorber life. Everything being a balancing act, I don't want to push it too far.

    Also, I find little difference in mpg, raising/lowering the pressure through a few psi, so opt for modest raise.
     
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  19. MikeDee

    MikeDee Senior Member

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    There was a video posted on this forum on hypermiling. It specifically said do not inflate more than 40 psi, but did not give a reason.
     
  20. Stevevee

    Stevevee Active Member

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    Mine are now 40/38, and I notice the difference in ride from 38/36. Fine on smoother roads, not too bad on rougher roads. but when turning onto parking lot etc with difference in height, , the baby carriage ride and head jiggling back and forth is a bit much.

    Definitely coasts better on all roads, and easier to drive all around.
     
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