Tires: Tires & PSI, dodge carvan, hairline cracks

Question
Inside the drivers door plate of my 1999 Dodge Carvan Sport SE, the recommended cold tire PSI is 35lbs on both the front and back.  My actual tires reflect a maximum cold tire PSI of 45lbs.  A 10lbs difference.  With gasonline prices rising, I want to get the maximum performance.  Should I go with the auto maker recommend PSI or the tire maker recommened PSI, or somewhere in the middle?  Why the difference?  Was this not the problem with Ford Explorers and Firestone tires a few years back?  Also, my front tires are showing some "hairline" cracks/fissures along the sidewall.  Is that due to normal wear and tear, or is it a safety concern.  Please advise.

Many thanks.

Answer
Mar,

What is printed on the sidewall of the tire is the MAXIMUM cold pressure, as required by law.  This is NOT a recommendation by the tire manufacturer.  This figure represents the maximum pressure the tire was designed to accommodate and should not be exceeded (with a few exceptions).

What is printed on the door placard is the pressure specified by the vehicle manufactuurer.  That figure represents the pressure needed to carry the load the VEHICLE was designed to carry, plus some additional amount of capacity, sometimes referred to as the reserve capacity.  This is the figure you should use as a starting point to figure out what is appropriate for your situation.

It is this reserve capacity issue that was at the heart of the Ford / Firestone disagreement a few years back.  The essence of this arguement was that Ford specified a reserve load of Zero.  In other words, the tire's load carrying capacity at the inflation pressure specified by Ford was EXACTLY equal to the maximum load the vehicle was designed to carrying.  This figure did not take into account any possible overloading, or the issue that folks don't check their inflation pressures regularly and it is common for many tires to be well below the specified value.  

Firestone's position was that good engineeering ought to dictate at least some reserve capacity - that NONE is not good engineering.

Personally I use 3 to 5 above the placard value.  I get better fuel economy, better tire wear, better wet traction, better snow traction, better steering response, better tire durability, and only give up a bit of ride harshness.

Sounds like your goals align with my thinking.

The hairline cracks are fairly normal, depending on how large they are.  Tires age, just like people do, and the rubber "dries out".  The cracks are a way of telling you this is happening and a way to tell if it is time to replace your tires for reasons other than wear.

My currrent recommendation is that tires should be replaced when they reach 10 years of age.  However, you should use 6 years of age if you live in the following states - CA, AZ, NV, NM, TX, and FL.  This is a temperature dependent thing.  The value is proportional to how far north you live - So TN would be 8 years.

Hope this helps.