Max Tire Pressure - Hot or Cold?

Discussion in 'General' started by Alanbhope, Jun 12, 2008.

  1. Alanbhope

    Alanbhope Member

    Hi All,

    I want to set my tire pressure at the manufacturer's highest maximum rating. Is that at cold (rested overnight) or hot (at the end of 30 miles of freeway commute?

  2. JamesBurke

    JamesBurke Active Member

    It's Cold or really ~70' f. The max pressure ratings are federally mandated. For type P passenger tires there are 3 grades, 35psi, 44psi, and 51psi.
  3. A024523

    A024523 Currently in Training

    Agreed with cold. My manual warns to not measure it hot, and I noticed mine reads 3-4 psi higher after highway speeds on a hot day.
  4. vtec-e

    vtec-e Celtic MPG Warrior

    I was thinking about this recently after my service engineer was crying at me about my tire pressure. But it got me thinking. If i have my tires at sidewall when cold, they'll be over that when warm. So IF i wanted to have a clear conscience with my tire pressure and never go above sidewall, would i be right in measuring my pressure while the tire was hot. It would mean the tire was below sidewall when cold but it would warm up quickly and not go above sidewall thereafter. I know none of this matters if i want to go above sidewall or want to be at the placard pressure. It only really applies to sidewall pressure.
    Then again, i could be looking into this way too much!

  5. Tochatihu

    Tochatihu Well-Known Member

    The max psi on the sidewall is a cold number. Driving will cause it to increase, which is entirely normal. It is also useful to also measure 'hot' psi especially after high speed, aggressive cornering runs and heavily poaded vehicle. If the hot psi is more than 10 percent higher than the cold, something needs to change.

    One optionis to increase the cold inflation pressure. This will reduce sidewall flexing and the associated heating. Next option is to reduce vehicle load, speed, or exciting cornering and braking style. If none of that is enough, you can conclude that your tires are not adequate for the task you've assigned them, and begin the search for better ones (typically with a high load capacity).

  6. bomber991

    bomber991 Well-Known Member

    Ah, I was wondering about that. Does the PSI depend on the speed rating of the tire or what? Like Z or W or whatever it is.
  7. Alanbhope

    Alanbhope Member

    Thanks, gang,

    I'm new to hypermiling - I'm in the current ScanGauge buying group - and while my tires' maximum sidewall rating is 51 psi, I was concerned that if I raised them to that pressure when they were cold I could be creating a dangerous condition when they heated up.

    Your help here is much appreciated, and I expect it will contribute to a few extra mpg.

    Thanks again,
  8. JamesBurke

    JamesBurke Active Member

    The only thing that I can see the PSI rating effecting is sidewall stiffness. imo. The infamous defective p235-75R-15's used on the Ford Explorer's were all 35 psi rated. (Firestone, Goodyear ect all had same problem). Very soft with lots of sidewall flex gave a smooth ride. These were all replaced with 44 psi rated designs. Very little sidewall flex with these and my "Mich" ltx's are like solid rubber. Makes the ride much harsher.

    The higher inflation pressure seems to reduce the increase in pressure because of reduced frictional heating from lower rolling resistance.
  9. wdb

    wdb semi-hyper

    (I sure wish people would give tire company engineers a little bit of credit, not to mention tire company lawyers. ;) )

    The PSI number on the sidewall is a COLD value. In addition, it represents the maximum pressure recommended by the tire manufacturer when the tire is at maximum load. So it's a very very safe number.
  10. Blaster94

    Blaster94 Well-Known Member

    The COLD pressure is rated at 32F, 0C . So at 70 degrees the pressure inside the tire will be higher than the cold pressure. This is normal and is engineered this way. You will not be creating a dangerous condition. You will notice an improvement in you mpg and maybe a bit stiffer ride.
  11. 97PROTEGE

    97PROTEGE Well-Known Member

    Hello Alanbhope,
    I was sketical at first about tire pressure.
    I am slowly, every 2 weeks increasing the PSI by 2 Lbs.
    Possibly this will work for you.
  12. MooingLizard

    MooingLizard Well-Known Member

    The only PSI value that I could find on my tires was 35 psi, "under standard loads". Given that 98% of my driving is only me commuting to/from work or carrying a few groceries in the trunk, is there another value for light loads that I should be using, or am I stuck with 35 psi? In a few days I'm planning to pump up the tires to the limit posted on them (kinda disappointed after checking them that it appears to be only 35 psi) plus a couple pounds.

    Finally, as a rule of thumb, do tires with higher PSI get better mileage? In other words, when I'm shopping for my next set of tires and choosing between two similar tires, should I choose the tires with a higher PSI rating?

  13. HemiSync

    HemiSync Well-Known Member

    I am not 100% sure of this but from the conversations I have heard on here about max sidewall pressures, usually lower max psi is because it is a less expensive tire.

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