Tires: are Plugs safe?, cross country trip, underinflated tires

Question
QUESTION: Recently on a cross-country trip we had a flat tire. Thanks to a warning light, I caught it before it was completely flat, and following the hiss I found the hole.
I put on the "banana" spare, but since we were 200 miles from home I knew it wouldn't get us there.  So I bought a $7 tire plug kit from Wal-Mart, and fixed the original tire in the parking lot.  It held all the way home, and 2 days later is still holding pressure.  Now I'm trying to find out if this fix is good enough, or if I need to replace the whole tire.

Local tire repair shops won't repair the tire because it has been plugged, and/or the injury is on the shoulder of the tread.  The hole itself is about half an inch up the sidewall, on the backside, and is small, taking about 5 minutes to drop the tire pressure from 35 to 30 psi. (Or at least it WAS small before I reamed the hole for the repair.) The plug kit consisted of a 3/16" reamer, a plugger that looked like a giant needle, and a sticky black cord.  There was no cement involved.  The plug sat (under pressure) for maybe 15 minutes before we drove on it, and I'm sure it got plenty hot on the 200 mile trip home. I would estimate the tire is about halfway through its treadlife, based on tread thickness.

I've gotten various opinions on the plug, some saying the flexing of the shoulder will eventually spit out the plug, others saying it will "vulcanize", and should last as long as the tire.

So can I trust it, or should I replace the whole tire?

ANSWER: Aaron,

The question you are asking is:  "If a tire holds air is it safe?"

The answer is no!!!!

If a tire has a separation - and it still holds air - was it safe?  The obvious answer is no - and the obvious reason is that underinflated tires can separate just due to the underinflation.

And the location pf the repair is important - sidewalls have very stange and complex movements, so repairs in this area tend to develop problems.  But a lot of folks who do repairs don't ever see their work again, so they think it is OK.  Is 1 in 100 OK?  I don't think so!   More like one in 10,000.  And if you are one of those 1 in 10,000, if you have a problem at high speed - well, it's hard to argue when you are in  a coffin.

So I think you need to replace the tire.



---------- FOLLOW-UP ----------

QUESTION: Thanks much.
Assuming I replace the tire (a rear tire), do I then need to replace the other rear - or the other three?  In other words: is it advisable to have one new tire and 3 used tires?

Answer
Aaron,

It is always best to have 4 tires the same, and 2 of the same would be next best, and changing 1 is least best.  This becomes a matter of how much risk you are willing take.

BTW, the risk is that the vehicle will tend topivot around the odd tire under severe braking.