<VV> High Tire Pressures - long (sorry)

[Dale Dewald]

Gentlemen;

See notes in text.

At 21:27 10/15/2008 -0400, "Ron" <ronh at owt.com> wrote:
>I don't understand the fetish for high tire pressures as GM steadfastly 
>maintained that 15 and 26 were the correct pressures.

This specification was for the 6.50-13 and 7.00-13 bias ply tires available 
at that time.  These were rated for maximum load at an inflation pressure 
of 30 psi.  Most modern radials, including the Firestone 380's, are 
designed for slightly higher inflation pressures at maximum load, in this 
case 35 psi. Therefore, it only logical to adjust the original factory 
specification to account for the change in tire technology, and inflate 
modern radials 5 psi higher--thus the recommendation of approximately 20 
front 32 rear for a car driven in "normal" conditions.

>   Even my modern Solara specifies 28 for any speed up to 100 and then 
> raises it to 36 only if driving continuously at over 100.  Has anyone 
> been driving their Corvair continuously, like all afternoon, at over 
> 100?  I'll stick to my 18 and 28 and consider it only mildly over pressured.

The Corvair owner's manual also recommends an increase in tire pressure for 
situations of "Continuous Service with up to full load capacity or high 
speeds." This is similar language to that used to describe service 
intervals and oil changes.  Since very few owners' driving habits and 
conditions meet the definition of "Standard Service," it is logical to also 
adopt the more severe service specifications for tire care. (Consider the 
highways and traffic we have now, compared to the '60s.)  In this case, the 
recommended tire pressures are 19 psi front, 30 psi rear (tire max).  If we 
take into account the adjustment for radial tire design, we arrive at the 
typical recommendation for daily driven Corvairs that has served us so well 
for the past 25 years; 24 psi front, 35 psi rear.  Note that GM recommends 
a minimum 11 psi difference front to rear.

At 00:49 10/16/2008 -0400, Richard/Gail Spiess <spiessdg at yahoo.com> wrote:
>Pardon me if I seem confused by all of the talk about tire pressure.  What 
>is wrong with using the pressures indicated in the owner's manual?  15 and 
>26 works for me.  Also, I just had new tires installed and they were 
>inflated with nitrogen.  No charge so why not.

The greatest advantage for using nitrogen in tires is that it is 
dry.  Moisture, as in water vapor, has a large change in vapor pressure 
between 0 and 40C, thus having a greater effect on inflation pressure 
between cold and hot than pure N2.  This is really not much of an issue if 
dry air is used for inflating tires in the first place.

An additional note: Many new passenger car tires are designed for even 
higher inflation pressures, with a rated maximum of 44 or 51 psi, and are 
designed for use at proportionally higher pressures.  It is prudent to 
check the sidewall rating of any new tires and make appropriate adjustments.

For example, my wife's daily driver 65 Monza is fitted with P195/70R14 
tires that have a 44 psi rating.  I have been running these at 28 psi 
front, 41 psi rear for the past 8 years that they have been on the 
car.  Handling and ride have been great. Wear has been perfectly even 
across the tread, to the point that she will need to get a new set next 
spring.  If I tried to run these at 15 front, 26 rear, the car would 
practically be riding on the rims and the tires would be destroyed in short 
order.

Dale Dewald
Hancock, MI