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Old 06-19-2015, 10:30 PM   #1
Join Date: Oct 2014
Posts: 34
Tire. Pressure

Taking a little survey, since most of you are much more experienced than I. We have a 405FK and are leaving right after 7/4 for an 8 week cross country vacation. I just installed a TPMS and that has me wondering about both tire pressures and alarm settings for the tpms. Suggestions?

Bill & Beth
Hyde Park, NY

2014 Sportscoach Cross Country 405FK
2015 GMC Acadia
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Old 06-20-2015, 04:58 AM   #2
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Then first you have to determine the pressure needed for the loads on tires , in the 98% of your use with a comfortable reserve but not that much that comfort and gripp gets lost.

This advice is for 18 degr C/65 degr F cold measured pressure , so measured when inside tire temperature = outside tire temperature at that 65 dgr F.

Then once you determined all this for all the axles on the car or combination, you can set lowest alarm , and I can calculate that for you, at what the pressures would be when coldest ambiŽnt temperatures for your surroundings.
Lets state that at zero degrees C/32 degr F for instance.

Highest border you then should even put as high as possible, because at sertain conditions the temperature in the tire can even rise to boiling point of water , 100 degr C/212 degr F ( severe braking when descending from mountains for instance).
With that higher temp the pressure rises too, can be as high as 40% when filled at 32degrF and temp rises to that 212 degr F.

You can also play al little with the basic setting to get your alarm borders at a better point.
Example: if determined pressure is 80 psi and high alarm at 20%, low alarm 20 % . then low alarm at at 64 psi and high at 96 psi.
Then try to get low alarm at 70 psi and high as high as possible by setting basic pressure not at 80 psi but at 87 psi.
then low alarm 20 % lower will be 69.6 psi and high alarm 104.4 psi.

All yust an example , it all depends on the borders you can set for your TMPS. And I did only estimation,calculating it will be more exact.

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Old 06-20-2015, 06:37 AM   #3
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I run LR D tires which have a max. cold pressure of 65# and I set my TST TPMS high alarm @ #78 (20%) and the low alarm @ 60#(8%), as I want to know asap if a tire is going down. I leave the high temp set at factory default.
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Old 07-16-2015, 06:36 PM   #4
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Originally Posted by jadatis View Post
This advice is for 18 degr C/65 degr F cold measured pressure , so measured when inside tire temperature = outside tire temperature at that 65 dgr F.


I don't understand this reference to "65F cold measured pressure" Is this some reference to a "standard laboratory conditions"?

Some people confuse "Ambient Temperature" which is the temperature of the outside air, with the theoretical "laboratory Standard" for doing chemical research.

As you know, or should know, tire pressure is set when the tire is at Ambient. This means not heated from being driven on for the previous few hours or being heated from being warmed by direct sunlight.
There is no adjustment for a difference between the actual Ambient temperature to some theoretical laboratory conditions.
Write a blog on RV tire application 40 years experience as tire design engineer. Freelander 23QB on Chevy chassis is my RV. Giving seminars on RV Tire applications (not selling) in Elkhart May 17-19, Allegan MI June 6-9, & Gillette WY July 18-21 RV Tire Safety Blog
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Old 07-16-2015, 07:53 PM   #5
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On my placard, it says 100 psi.

I weighed the MH front and rear, then looked up the tire weights on the tire pressure table.

The weight on all 6 of my tires is below the minimum in the table, so the absolute minimum pressure for my tires is 70 psi. I chose to run 80 psi in them. This makes for a little smoother ride compared to the 100 psi called for on the coach placard. I also put 50 psi in my air bags, this also help smooth out the ride.

I don't have TPMS, but if I did, I'd probably set mine at 70-75
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pressure, tire

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