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Re: Tire Pressure Mystery
Posted by Argisltd (more from Argisltd) on Tue, 1 Oct 2002 15:32:58
In Reply to: , Kok Chen, Sat, 17 Dec 1988 12:00:00 

Just one comment on tire pressure.
   Those ratings and recommendations for tire pressure are for COLD readings, 
and at a specific load.
Tires can easily pick up 4-6 psi (and more) when run for a long period of 
time at high speed. That's for (mostly) LEGAL speeds. I've picked up 2 PSI 
just driving local roads (with stops) from home to the gas station 3 miles 
away.
   Those pressures are for STP conditions which nominally is 15C (60F).
PV=nRT means that you can gain and lose a few pounds depending on when you 
measure it.
   One thing to keep in mind is that, if you check your tires on a reasonable 
schedule and keep them at mfr's recommendation plus a few pounds, more 
important than the absolute pressure is the variation between them. That can 
have a big effect on handling.

BTW, I checked the warning on the visor on my '86 900S.
The recommended tires pressures are:
1-3 passengers: 30PSI (2.1 bar)
4-5 passengers: 35 PSI (2.4 bar)
at a not to exceed speed of 130MPH
(I don't think I'd be hauling 5 people or equivalent cargo at 130MPH ;
with a design tire of 115/60R15.
  Does anyone actually pay any attention to the mfr's recommendation?

  I inflate the tires to 32PSI stone cold at home, 34 if I'm at the gas 
station, and I figure the operating pressure when running down the highway is 
about 38PSI.
All measured with the mfr's pressure gauge that came with the car ;-).

Anyway, more important than the absolute pressure (assuming that you're 
somewhere in the ballpark) is maintaining equal pressure all around and 
checking it frequently.
/set/nanny-mode=off ;-)

BTW:
My old '69 Dodge Charger had a recommended tire pressure of 24 PSI ;-)

Bob S
'69 Dodge Charger, 14' bias ply tires at 24PSI ;-)
'78 99 Turbo, sold at 149K for a ...
'86 900S, 300K
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