[Healeys] Conclave tire air pressure change trivia

Bob Spidell bspidell at comcast.net
Mon May 24 10:16:09 MDT 2021

Interesting point. I'm familiar with three types of tire pressure 
gauges: 1) the 'pencil' type with a sliding 'ruler', 2) the analog type 
which usually use a Bourdon tube (used in our 'safety' gauges) and 3) 

I can't imagine ambient pressure affecting the pencil/slider type; a 
Bourdon tube could be slightly affected by temperature I suppose, and 
the digital gauges usually use a pressure/load sensor (like digital 
scales) and I've not noticed a port to compare with ambient. So, I'm 
gonna go out on a limb and say gauges measure absolute--not 
differential--pressure so the ambient/tire delta is a non-factor (but I 
have no proof).

On 5/24/2021 9:05 AM, Bob Haskell wrote:
> 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.
> Cheers,
> 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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