Tires: Load index, age, and lack of use

Ever since my roadtrip to/from Los Angeles, I noticed an intermittent vibration with the Popemobile (with the steering wheel and the car itself).  I suspected that the potholed streets of Oakland and nearby freeways (I'm looking at you, Oakland section of Interstate 880!) had slowly destroyed a suspension bushing or two.

An inspection by the local independent shop revealed that the suspension was fine.  The mechanic asked me if I knew that one of my tires had a load index of 99.  I did not.  Did I also know that my car is supposed to have tires with a load index of 103?  No, I did not.  I did not even know what a load index was last week.

This is a shot of the door jamb sticker.  I suggest that you look at it again.  Previously, I've only paid attention to the size of the tires and the recommended pressure.  I didn't know the number (103) after the tire size was the load index.  According to this chart, a 103 rated tire has a load capacity of 1,929 pounds (875 kilos).

When the previous owner of the Phaeton suffered a flat, he replaced it (probably inadvertently) with a Continental tire with just a 99 load index, which has a capacity of only 1,709 pounds (775 kilos).  Given the mass of the Phaeton, the sidewalls on the weak tire had been slowly warping and collapsing.  This explained a lot of the vibration.

And since the other three tires were going bald, I ordered a full set of Continental ExtremeContact DWSs from Tire Rack.  They were installed today.

But while I was waiting for the install, I realized something.  Three of the four tires came with the car when it was originally purchased in 2005.  They must be seven to eight years old!  They were never changed because the car did not have that many miles and they still had tread left.  But with the dry and hot climate (the car was in Arizona from 2005 to 2011) and infrequency of use (the car was driven twice a month by the previous owner), the three tires were probably deformed.  Why didn't I think of this earlier?  What a dope I am.  This probably also contributed to the vibration.

You know how when you are used to seeing something everyday, you don't notice that there is something wrong with it?  Well, the old tires were bald.  But I didn't realize how bald they were until I saw the new tires on my car.  By comparison, the new Contis are like knobby off-road tires.  I briefly took it on the freeway.  The ride was beyond perfect.  What a difference!  (I hope it's not a placebo effect.)