[Healeys] Conclave tire air pressure change trivia

Bob Haskell rchaskell at earthlink.net
Mon May 24 10:05:15 MDT 2021

Atmospheric pressure at sea level, 59 degF is 14.7 psia.  At 6,700 ft 
and 59 degF, the pressure is 11.47 psia.  If the tire gage is measuring 
gauge pressure, the delta between the pressure in the tire and 
atmospheric pressure, the change in atmospheric would account for most 
of the difference seen.  But I have no idea if the tire gauge used works 
that way or not.


Bob Haskell
Austin Healey 3000 BN7/BT7 registrar

On 5/24/21 11:36 AM, Bob Spidell via Healeys wrote:
> I for one can't think of an explanation for this startling phenomenon. 
> Obviously, the ambient air pressure is less at altitude--known as 
> 'adiabatic lapse rate' to nerds and pilots--which, theoretically could 
> cause the tires to expand, increasing their volume and lowering the 
> pressure (according to the Ideal Gas Law). But, I doubt steel-belted 
> radial tires expand or contract much, but air pressure in fixed volume 
> goes up about 1psi per 10degF IIRC.
> Was there a 40degF difference in temperature between Portland and Big 
> Bear (California has been pretty warm the last few weeks, which I can 
> attest to since our A/C is on the fritz)? Also, temps vary when tires 
> are warmed by the sun or friction with the road, so if I can't check 
> pressure completely in the shade I'll under-fill the tires in the sun 
> by half a degree. So, my guess is a significant difference in air 
> temps, and asphalt absorbs a lot of heat which gets transferred to the 
> tires (Portland = (often) cloudy and cool; SoCal = (often) sunny and hot).
> On 5/24/2021 8:20 AM, Jean Caron via Healeys wrote:
>> Sounds like you have one of those rare wheels with self-adjusting 
>> pressure, I wonder when these became an option and on what model 
>> !!!!!! LOL!
>> Jean
>> Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986> for 
>> Windows 10
>> *From: *richard mayor <mailto:boyracer466 at gmail.com>
>> *Sent: *May 24, 2021 9:28 AM
>> *To: *Healeys <mailto:healeys at autox.team.net>
>> *Subject: *[Healeys] Conclave tire air pressure change trivia
>> My Healey tire pressures went from 29 psi here in Portland, Oregon, 
>> to 33 psi at Big Bear Resort in California - 6,700 feet above sea 
>> level.  I then lowered my tire pressures to 29 psi.
>> Checked my tires today back in Portland - 70 feet above sea level and 
>> all tires at 25 psi.
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