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Technical Technical discussion on HW and SW mods with the intent of increasing performance and/or fuel economy.

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Max Tire Pressure - Hot or Cold?

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Old 06-12-2008, 04:29 PM
Alanbhope Alanbhope is offline
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Max Tire Pressure - Hot or Cold?

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?

Thanks,
Alan
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Old 06-12-2008, 05:19 PM
JamesBurke JamesBurke is offline
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Re: Max Tire Pressure - Hot or Cold?

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.
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Old 06-12-2008, 05:39 PM
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A024523 A024523 is offline
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Re: Max Tire Pressure - Hot or Cold?

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.
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Old 06-12-2008, 08:32 PM
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vtec-e vtec-e is offline
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Re: Max Tire Pressure - Hot or Cold?

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!

ollie
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Old 06-12-2008, 11:51 PM
Tochatihu Tochatihu is offline
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Re: Max Tire Pressure - Hot or Cold?

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).

DAS
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Old 06-13-2008, 12:18 AM
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Re: Max Tire Pressure - Hot or Cold?

Quote:
Originally Posted by JamesBurke View Post
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.
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.
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Old 06-13-2008, 07:18 AM
Alanbhope Alanbhope is offline
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Re: Max Tire Pressure - Hot or Cold?

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,
Alan
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Old 06-13-2008, 08:57 AM
JamesBurke JamesBurke is offline
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Re: Max Tire Pressure - Hot or Cold?

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.
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Old 06-21-2008, 08:56 PM
wdb wdb is offline
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Re: Max Tire Pressure - Hot or Cold?

(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.
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Old 06-22-2008, 09:33 AM
Blaster94 Blaster94 is offline
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Re: Max Tire Pressure - Hot or Cold?

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.
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