Tires: Tire Inflation, proper inflation pressure, passenger car tires

Question
I've read the history, and I know you get asked this question a lot, but I want to make sure I'm correct.  I recently bought tires for a 2001 Oldsmobile Alero size p225/50/16 Sumito brand and the max air pressure is 51 psi.  The plackerd on the car says 35 psi.  If I have read past responses correctly, I should carry 35 psi in the tires correct?  I thought that you always wanted to run 1~2 psi under the max load rating, so you know your not underinflated.  What is the correct psi to run in the new tires.

Answer
Donnie,

I think you already know the answer I'm about to give:

If you are using the same tire size as what is listed on the placard, then the pressure listed on the placard is appropriate.  Having said that, I usually suggest that folks run 3 to 5 psi above that value to gain some important performance improvements - which you have already read about.

But obviously I need to explain why you don't look to the sidewall of the tires for information about proper inflation pressure.

First, what is written on the sidewall of a tire is the result of someone reading the government regulation on the subject.  Unfortunately, the regulation is a little unclear.  It reads something like this:  "The sidewall shall have imprinted the maximum load carrying capacity and pressure."

Does this mean the maximum usable pressure or does it mean the pressure where the maximum load carrying capacity occurs?  These are 2 distinctly different pressures.

For example - and I'm going to use passenger car tires here although it applies to other types of tires as well - The maximum load carrying capacity for a P225/50R16 SL is 1367 # and that occurs at 35 psi.  However, it is permissible, and even recommended, that additional pressure be used if the vehicle is to be operated at high speeds.  So 44 and 51 psi are "standardized" by the tire standardizing organizations as what should be used for certain circumstances - like high speed.

So......What should be written on the sidewall?  Well, for a P225/50R16 SL you could use 35, 44 or 51 psi, depending on how you read the regulation and the published tire standard.

And as a result you can't go by what is written on the sidewall.

I should mention that if you look on the sidewall of your Sumitomo tires, you will probably see a different load carrying capacity that what I have written above.  The tire will probably say 630 kg - which is the same as 1389 lbs.  This is because this tire is being designed to a Japanese tire standard.  

OK - see how complicated this is?  That's why the placard info is so vital and why there is a government regulation requiring that it be on every vehicle sold in the US.