<VV> Tire Pressure (Kinda Corvair content)

[Dale Dewald]

At 04:45 PM 4/15/06 -0400, Kirk Eck wrote:

>If I put larger or for that matter even smaller tires
>on my car how do I determine the tire pressure I
>should run?  I know on a vair the front should have
>less pressure than the back but how much is enough, if
>say all 4 tires have a maximum of 44 PSI?

As a general rule you should run the highest inflation pressure possible to 
get best fuel economy, but not so much that vehicle ride and handling are 
affected (safety) or that excessive wear occurs.  If you place larger or 
smaller tires on a vehicle, some adjustment to inflation pressures might 
have to be may be made based on the load carrying capacity of the tires.  A 
larger tire with greater load capacity might perform best with 
proportionally less inflation pressure than the stock size, but you will 
have to experiment to be sure.

For the specific situation of the Corvair (car), the rear tires need to run 
pretty close to their maximum inflation pressure and the front tires about 
10 PSI less.  For the most common stock radial replacement sizes, 
P175/80R13 or P185/80R13 with 35 PSI max, this typically works out to 20-25 
PSI front and 30-35 PSI rear.  These same pressures also work well with 
standard load P185/70R14 or P195/70R14 tires (much greater selection, but 
need 14" wheels) that are the same OD as the stock tires.

It seems that you are considering a set of speed rated tires for your 
Corvair which often have higher maximum inflation pressures than standard 
tires.  There is such a set on my wife's '65 Monza: Pirelli P4000 
195/70R14's rated for maximum load at 44 PSI.  In this case inflation 
pressures are adjusted upward in correspondence with the tires' maximum 
inflation pressure. I keep the tires on her car at 30 PSI front and 42 PSI 
rear.

>   On a new
>car if I beef up the tires what do i do to detemine
>the correct pressure?

For best balance of fuel economy, tire life and handling safety it is best 
to run the manufacturers recommended tire pressures.  I assume that this 
question regards installation of larger tires that have the same maximum 
inflation pressure (not recommended on newer cars because it interferes 
with vehicle speed calculations for the engine management computer).  In 
such case I would use the same pressures as used for the original tires but 
take note of tire wear and make adjustments as necessary.  If the new tires 
have a higher maximum inflation pressure (but same load capacity) then 
simply increase the running pressures proportional to the difference in max 
inflation (like the example above).

Clear as mud, eh?

If you have a particular tire or application in mind, please e-mail me 
off-list and I will try to help you out.

Dale Dewald
Hancock, MI


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