Foretravel Owners' Forum For owners and Fans of Foretravel Motorcoaches September 10, 2025 ...

Tire Pressure

October 13, 2022, 09:16:40 pm

I know that this topic brings in a ton of "discussion" about what the correct air pressure should be, but I do have a couple of questions.

(My mind wanders when driving down the highway, and I come up with a lot of dumb questions and ideas!)

1. If starting at say........95 PSI, and the ambient temperature is 75 degrees, after driving down the highway for 100 miles with the tires fully warmed up, what is an acceptable raise in pressure and temperature?  Watching my TPMS, I have seen the pressures raise as much as 15 PSI and 20 degrees, depending on the outside temperature.  Just curious.

2. When setting the pressure, again say........95 PSI cold, but starting at an altitude of 9500 feet, should some sort of pressure accommodation be made compared to Sea Level?

I am sure there will be many opinions, just curious.

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Re: Tire Pressure

Reply #1 – October 13, 2022, 10:22:14 pm

Your are correct,  90# at 60* is a lot more air than 90# @  90*. 
 Nobody seems to care. Set it at your cold value and run it . 
 My road race car has bleed offs. So, the first turn of the first lap , the tire has a good foot print regardless of temp.  IT builds about 12#  by the end of the 30 min race.    But the bleeders maintain the correct pressure. 

 MY TPS shows the sunny side of the bus a couple of pounds higher than the cold side .  Yes, I leave at 94 front and the alrm starts at 100.  It will reach 100.

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Reply #2 – October 14, 2022, 06:55:50 am

MarkC

1.  That's normal, ours does the same, I set pressures "cold" meaning before a trip, before running it down the highway. It may be 65* ambient or it may be 85*, I don't worry about it. Just set your pressures to the tire manufactures' recommended  pressure according to your axle weight on their chart.

2. Set you pressure at the altitude you will be driving most, don't stress over the altitude changes along the way.

Unless you are running at max pressure and weight rating of the tires based on your weight. I wouldn't be concerned over the pressure rise from cold to hot. Even then, not stressed over it. 😎

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Reply #3 – October 14, 2022, 08:07:38 am

Yup, altitude has a rather insignificant effect on tire pressure.  But, often, higher elevations are also cooler-- heck, that is why we head for the mountains in the summer.  I know I have to add air to every vehicle every time when going from Texas or Florida to Angel Fire NM.

Temperature has a much more dramatic effect.

And, cold, as defined by all tire manufacturers is "at ambient temperature before driving or outside warming forces like sitting in the sun".  The engineers who formulated the Pressure Charts are fully aware of the Ideal Gas Law and know what change will occur with the heat of the day and heat generated by driving.

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Reply #4 – October 14, 2022, 07:03:14 pm

Any guesses on pressure in altitude.  Living at 9000 feet, my trips always start in that altitude.  By the time I get down the hill to 5000 feet, (it's a 30 mile long winding highway) my tires are hot. 
If driving 300 to 400 miles before stopping overnight to cool down again to check, I wonder if it's an issue.

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Reply #5 – October 14, 2022, 08:06:50 pm

Makes not a bit of difference. If you have your tires at the pressure for the weight or more (pressure), that's all you need. Either the tires are getting radiant heat from the brakes or they are under inflated and heating because of that. Tires are usually warm as they flex a lot in the contact area when underway. The lower the pressure, the more they flex.

Ambient drops 3.5F for every thousand feet above sea level. So, it's cooler up high which more than makes up for any difference in tire expansion from the difference in outside and inside tire/air pressure.

Pierce

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Reply #6 – October 14, 2022, 09:03:57 pm

Yup, elevation is not meaningful in terms of PSI change.

Temperature IS.

Often high elevation is also cooler, so it appears to have an impact.

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